Enhancement of the performance of planar perovskite solar cells by active-layer surface/interface modification with optimal mixed solvent-antisolvent post-treatment

Modification of perovskite films surface/interface through the solvent vapor post-treatment during the film annealing can significantly improve the morphology and crystallinity of the perovskite film, thus enhances the efficiency of the perovskite solar cells (PSCs). In this work, we used a solvent-antisolvent mixture of Dimethyl sulfoxide (DMSO) and chlorobenzene (CB) in the post-treatment of perovskite films during device fabrication to achieve a high-power conversion efficiency of 19.15% in planar perovskite solar cells. The use of chlorobenzene as an additive to DMSO in the post-treatment of the perovskite films was shown to optimize the its morphology and resulted in films with highly fused grains. The modified perovskite film surface not only showed a decreased number of pin-hole and trap density at the surface, but also an increase in the charge transfer at the interfaces and reduced the susceptibility to low-frequency interface polarization. Furthermore, the impedance spectroscopy and I–V characteristics of the electron-only PSC devices also verified the conclusions above. Overall, this work demonstrates mixed solvent-antisolvent post-treatments of perovskite films as an effective modification strategy to tune their surface/interface properties. This approach is anticipated to be extrapolated to other categories of polycrystalline bulk materials and devices.Agency for Science, Technology and Research (A*STAR)Nanyang Technological UniversityThe authors acknowledge Nanyang technological University (NTU), and Agency for Science, Technology and Research (A*STAR) for sponsoring this research

Determination of dominant recombination site in perovskite solar cells through illumination-side-dependent impedance spectroscopy

Perovskite solar cells (PSCs) have attracted wide attention due to their capacity to achieve high-power conversion efficiencies. However, the high trap-assisted recombination taking place in the active layer leads to performance loss in PSCs. In particular, the excessive recombination at the interface between the perovskite active layer and the carrier selective contacts can be especially problematic. Therefore, the identification of the dominant recombination pathways in a given PSC architecture is of significant importance for the mitigation of losses and enhancement of device performance. Here, we introduce an approach for identifying the dominant recombination pathways in PSCs by applying illumination-side-dependent impedance spectroscopy (ISD-IS) measurements on the devices with a semi-transparent top electrode. We validate this technique using coupled ionic-electronic numerical simulations and apply it experimentally on a standard PSC structure. Overall, this approach could be of significant importance for pinpointing the performance bottlenecks in PSC devices under operationally relevant conditions and providing a more detailed picture of the losses in a complete PSC device by examining its behaviors under illumination from both sides at different operation conditions, which could allow for a more targeted optimization strategy of PSCs to improve their performance.Agency for Science, Technology and Research (A*STAR)Nanyang Technological UniversityThe authors acknowledge Nanyang technological University (NTU) and Agency for Science, Technology and Research (A*STAR) for sponsoring this research

Low-temperature processed, stable n-i-p perovskite solar cells with indene-C60-bisadduct as electron transport material

Organo-metallic halide perovskites (OMHP) have proven to be promising light absorbers with superb optoelectronic properties for developing the next generation of low-cost solar cells. Over the past years, the extensive research efforts on perovskite solar cells (PSCs) have led to an impressive improvement in the photovoltaic performance on many fronts and have their main field of applications in low-temperature and low power consumption photo-electronic devices, However, a wide range of highly performing PSCs structures involves the use of metal oxide electron transport materials (ETMs) such as TiO2 which requires high processing temperature that could result in a higher manufacturing energy input and cost. This also could hinder the development of low-cost and low-temperature scalable processes for device fabrication on rigid or flexible substrates. Here, we develop a low-temperature procedure (below 100 °C) that make use of Indene-C60 Bisadduct (ICBA) as an alternative ETM in the planar n-i-p-structured PSCs. After modifying the ICBA layer, we not only improved the optimum performance and stability of the device, but also study its influence on the device operation using impedance spectroscopy, and finally achieved a stabilized power conversion efficiency of 13.5%. Thereby, this study will establish low-temperature ETM as an outstanding candidate for future high stability PSCs production due to its high performance, low process temperature and easy fabrication.Ministry of Education (MOE)The authors acknowledge the Ministry of education (MOE) of Singapore for sponsoring this research [Grant Number RG176/16]

Enhancement Of The Performance Of Organic Solar Cells By Electrospray Deposition With Optimal Solvent System

Electrospray (ES) as a thin film deposition method that is uniquely suited for manufacturing organic photovoltaic cells (OPVs) with desired characteristics of atmospheric pressure fabrication, roll-to-roll compatibility, less material loss, and possible self-organized nanostructures. The additional solvent with high electrical conductivity plays an important role in ES deposition process to fabricate OPVs with active layer composed of polymer mixture poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PC 61BM). Here we introduced acetic acid, which possesses high electrical conductivity, as additive solvent in ES process. The dependence of device performance on the concentration of acetic acid was investigated, and optimal ratio was obtained. To further demonstrate the influence of additive solvents with different electrical conductivity, OPV devices with active layer deposited by ES method using solutions containing acetic acid, acetone or acetonitrile were fabricated. The characteristics of active layers were revealed by optical microscope, atomic force microscopy, UV-vis spectroscopy and X-ray diffraction. Compared with additive solvents of acetone and acetonitrile, the active layer formed by electrospraying solvent containing acetic acid demonstrated enhanced vertical segregation distribution and improved P3HT crystallinity, which resulted in better device performance. OPV device using acetic acid as additive achieved power convention efficiency (PCE) of 2.99±0.08% under AM 1.5 solar simulation, which is on par with that of the spin coated device (PCE 3.12±0.07%). © 2013 Published by Elsevier B.V

Effects Of Damkhöler Number Of Evaporation On The Morphology Of Active Layer And The Performance Of Organic Heterojunction Solar Cells Fabricated By Electrospray Method

The electrospray (ES) process has emerged as a scalable and efficient fabrication method for organic photovoltaic cells (OPVs). However, the ES process could often involve numerous parameters, which impose a major challenge on uncovering the interplay among process, morphology of active layers, and device performance. This work attempts to reduce the parameter space and capture the essence of the ES process using the Damkhöler (Da) number of evaporation, which is the ratio of the droplet residence time over evaporation time. We first derived an explicit equation for Da that links nine different parameters affecting the process. Experimental results indicate that Da number shows strong effect on morphology and crystallinity of the active layer composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Most remarkably, the power conversion efficiency exhibits monotonically dependence on Da values spanning more than one order of magnitude, e.g., from 0.13 to 1.52. This finding suggests that Da number analysis can reduce the large parameter space in electrospray deposition, thus provides a simple way to control and predict the morphology of active layer and the performance of the solar cells

Soluble bipolar star-shaped molecule as highly stable and efficient blue light emitter

10.1039/c4ra14934cRSC Advances52015399-1540

Cesium Carbonate Functionalized Graphene Quantum Dots as Stable Electron-Selective Layer for Improvement of Inverted Polymer Solar Cells

Solution processable inverted bulk heterojunction (BHJ) polymer solar cells (PSCs) are promising alternatives to conventional silicon solar cells because of their low cost roll-to-roll production and flexible device applications. In this work, we demonstrated that Cs2CO3 functionalized graphene quantum dots (GQDs–Cs2CO3) could be used as efficient electron-selective layers in inverted PSCs. Compared with Cs2CO3 buffered devices, the GQDs–Cs2CO3 buffered devices show 56% improvement in power conversion efficiency, as well as 200% enhancement in stability, due to the better electron-extraction, suppression of leakage current, and inhibition of Cs+ ion diffusion at the buffer/polymer interface by GQDs–Cs2CO3. This work provides a thermal-annealing-free, solution-processable method for fabricating electron-selective layer in inverted PSCs, which should be beneficial for the future development of high performance all-solution-processed or roll-to-roll processed PSCs.ASTAR (Agency for Sci., Tech. and Research, S’pore

Elimination of Burn-in Open-Circuit Voltage Degradation by ZnO Surface Modification in Organic Solar Cells

Photodegradation of inverted organic solar cells based on ZnO as an electron transport layer (ETL) was studied over short time scales of 5 min and 8 h. Devices with ZnO as ETL reproducibly exhibited a steep loss of open-circuit voltage, <i>V</i>_OC, and shunt resistance, <i>R</i>_SH, in a matter of minutes upon illumination. Removing the UV-content of illumination minimized <i>V</i>_OC loss and impact on the device’s shunting behavior, indicating its role in the loss. Application of an ultrathin layer of Al on ZnO led to almost negligible photoinduced <i>V</i>_OC loss up to 8 h of exposure. By applying the fundamental Shockley diode equation, we approximated the <i>V</i>_OC loss to be caused by dramatic increases in reverse saturation current <i>I</i>₀. We attribute the increased rate of recombination to diminished carrier selectivity at the ZnO/organic interface. Devices with Al modified ZnO ETL demonstrated remarkable <i>R</i>_SH (1.4 kΩ cm² at 1 sun), rectification ratio (10⁶) and reverse saturation current density (2.1 × 10^–7 mA/cm²)

Interface passivation using choline acetate for efficient and stable planar perovskite solar cells

In order to enhance the efficiency and robustness of perovskite solar cells (PSCs), surface passivation is crucial to minimize surface defects, improve charge transfer, and inhibit the penetration of deteriorating agents. In this study, we demonstrate that choline acetate (ChAc) can effectively passivate the surfaces of perovskites to improve their stability and photovoltaic performance. The perovskite film passivated with ChAc shows many improvements, such as greater crystallinity, smoother surface topography, preferable alignment of energy levels, and lower defect density. As a result, the champion power conversion efficiency (PCE) for the pristine and ChAc PSCs is 18.20% and 19.80%, respectively. The passivated PSCs also display superior stability, as evidenced by retained unencapsulated PCE of 93% after 600 hours of storage at ambient conditions and 40% relative humidity at 25 °C, compared to 85% retained for pristine PSCs. Our results provide a straightforward and very efficient way to produce high-performing and stable PSCs.Energy Market Authority (EMA)Ministry of Education (MOE)Submitted/Accepted versionThe research is supported by the AcRF Tier2 grant (MOET2EP50121-0012) and AcRF Tier1 grant RG60/22 from the Singapore Ministry of Education, and the EMA-EP004-EKJGC-0003 grant from the Energy Market Authority (EMA) Singapore