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The hole transport material (HTM) free carbon based perovskite solar cells (C-PSCs) are promising for its manufactural simplicity, but they currently suffer from low power conversion efficiencies (PCE) largely because of the voltage loss. Here, a new strategy to increase the PCE by incorporating an ultrathin ferroelectric oxide PbTiO3 layer between the electron transport material and the halide perovskite is reported. The resulting C-PSCs have achieved PCEs up to 16.37%, which is the highest record for HTM-free C-PSCs to date, mainly ascribable to the ferroelectric layer enhanced open circuit voltage. Detail measurements and analysis show an enhanced built-in potential in the C-PSCs as well as suppression of the non-radiative recombination due to the ferroelectric PbTiO3 layer incorporation, accounting for the boosted V-OC and photovoltaic performance.

Over the last decade, perovskite solar cells (PSCs) have emerged as one of the most promising new photovoltaic technologies in terms of the rapidly rising efficiency and low-cost fabrication processes. The new record of certified power conversion efficiency (PCE) has reached 24.2% in 2019. However, the high-performance PSCs are seriously questioned on the stability issues. The use of vacuum deposition process of noble metal electrodes and employment of the expensive hole transport material (HTM) are high-cost and high energy consuming methods, also limiting their practical applications. In this respect, the HTM-free carbon-based PSCs (C-PSCs) are believed to be the front runner towards commercialization among all types of PSCs because of the long-term stability and low-cost advantages. However, the efficiency of C-PSCs is much lower and increases slower than conventional PSCs, steadily detracting their competitiveness among various kinds of PSCs. The issues in C-PSCs include the poor perovskite/carbon contact, weak hole selectivity, large VOC loss and trap-induced charge recombination, etc. They are undermining the photovoltaic performance of C-PSCs and are urgent to be solved. In my thesis research, I focused on the understanding and designing of C-PSCs, and developed several effective strategies to improve the performance and push the development of C-PSCs. Firstly, to improve the quality of perovskite/carbon contact, I developed a new deposition method named the ultrasound-spray deposition for carbon electrode (CNTs) with the embedded structured C-PSCs. This method employs pure CNT ink as carbon source, the naturally aligned CNT bundles by weak Van der Waals forces can collapse into individual nanotubes with the help of ultrasonic sound power and high surface tension of solvent, which ensures the smoothness and compactness of the CNT electrode. This deposition method achieved a high-quality carbon electrode and seamless contact at perovskite/carbon int

Carbon-based perovskite solar cells (C-PSCs) are prized for their simple device structure, high cost-efficiency and good stability. However, the absolute efficiency of C-PSCs is still low due to inappropriate C/perovskite interfaces. Herein, we report a simple ultrasound spray method to deposit pure multi-walled carbon nanotubes (MWCNT) films. When directly deposited on perovskite films, MWCNT could form a seamless contact at the perovskite/carbon interface, thus boosting the power-conversion efficiency (PCE) of C-PSCs to 14.07% as compared to drop cast MWCNT electrodes. In addition, when the perovskite surface was embedded with a sub-monolayer of nickel oxide nanoparticles (NiO NPs) prior to the MWCNT deposition, we achieved a champion efficiency as high as 15.80%, which is among the highest C-PSCs efficiencies reported to date. Detailed characterizations revealed that by planting NiO NPs into the surface region of CH3NH3PbI3 crystals, the hole extraction efficiency was effectively enhanced and interfacial recombination was reduced. With silicon dioxide (SiO2 NPs) as a control, the NiO NPs layer was found to favorably bend the energy levels at the interface for selective hole extraction, thereby enhancing the overall photovoltaic performance. The combination of ultrasound spray and nanoparticle embedment technologies opens the way to cost-efficient and scalable production of C-PSCs. © 2017 Elsevier Ltd

Bismuth vanadate (BiVO4) has been regarded as a promising photoanode material for photoelectrochemical (PEC) water splitting owing to its rich elemental abundance and relatively narrow bandgap. However, the incompatibility of the penetration depth and short diffusion length limits its performance. To overcome this shortcoming, we develop a cost-effective stamping method to fabricate inverse nanocone array (ICA) substrates for supporting nanoporous Mo-doped BiVO4 films. The ICAs show a remarkable light trapping effect in such a way that the intensive light absorption region is advantageously shifted from the top of the active layer on a planar substrate to the bottom surrounded by the ICA, where charge separation is strikingly more efficient. By integrating the ICA-photoanode with a tailor-made, bandgap-adjustable perovskite solar cell, we devised a PEC-photovoltaic (PEC-PV) tandem device, which has achieved a self-powered STH efficiency of around 6.3%. Our study opens a new avenue for designing solar fuel devices with PEC-PV architectures.

Compared to the conventional perovskite solar cells (PSCs) containing hole-transport materials (HTM), carbon materials based HTM-free PSCs (C-PSCs) have often suffered from inferior power conversion efficiencies (PCEs) arising at least partially from the inefficient hole extraction at the perovskite-carbon interface. Here, we show that boron (B) doping of multiwalled carbon nanotubes (B-MWNTs) electrodes are superior in enabling enhanced hole extraction and transport by increasing work function, carrier concentration, and conductivity of MWNTs. The C-PSCs prepared using the B-MWNTs as the counter electrodes to extract and transport hole carriers have achieved remarkably higher performances than that with the undoped MWNTs, with the resulting PCE being considerably improved from 10.70% (average of 9.58%) to 14.60% (average of 13.70%). Significantly, these cells show negligible hysteretic behavior. Moreover, by coating a thin layer of insulating aluminum oxide (Al2O3) on the mesoporous TiO2 film as a physical barrier to substantially reduce the charge losses, the PCE has been further pushed to 15.23% (average 14.20%). Finally, the impressive durability and stability of the prepared C-PSCs were also testified under various conditions, including long-term air exposure, heat treatment, and high humidity. © 2017 American Chemical Society.

Solvent engineering has been extensively used to control the growth of a high-quality perovskite layer for solar cells by forming intermediate phases. However, the intermediate phase formation is often poorly understood and its effects on the perovskite layer growth are still elusive. Here, we have conducted a systematic and in-depth study on the above two issues through a strict control over the DMSO/DMF ratio in CH3NH3PbI3 perovskite solutions, and thus an effective control over the compositions of intermediate films. The films thus obtained, including perovskite, perovskite/MA2Pb3I8(DMSO)2 and MA2Pb3I8(DMSO)2, afford perovskite crystals via down-growth, down-and up-growth, and up-growth mechanisms, respectively. Significantly, the up-growth perovskite crystals from the pure MA2Pb3I8(DMSO)2 exhibits the best interface contact with NiO substrate, optimal alignment without horizontal grain boundaries and a relatively large grain size, which facilitate charge transfer and reduce charge recombination in PSCs. As a result, the PIN planar PSCs based on NiO have achieved a PCE of 18.4%, a value which is among the highest for NiO-based PSCs, with the highest stability among the tested sample cells. Furthermore, the pure MA2Pb3I8(DMSO)2 intermediate phase presents a high long-term stability, which enlarges the operating window for perovskite deposition and thus considerably improves the device processibility. © 2017 Elsevier Ltd

2D halide perovskite materials have shown great advantages in terms of stability when applied in a photovoltaic device. However, the impediment of charge transport within the layered structure drags down the device performance. Here for the first time, a 3D–2D (MAPbI3-PEA2Pb2I4) graded perovskite interface is demonstrated with synergistic advantages. In addition to the significantly improved ambient stability, this graded combination modifies the interface energy level in such a way that reduces interface charge recombination, leading to an ultrahigh Voc at 1.17 V, a record for NiO-based p-i-n photovoltaic devices. Moreover, benefiting from the graded structure induced continuously upshifts energy level, the photovoltaic device attains a high Jsc of 21.80 mA cm−2 and a high fill factor of 0.78, resulting in an overall power conversion efficiency (PCE) of 19.89%. More importantly, it is showed that such a graded interface structure also suppresses ion migration in the device, accounting for its significantly enhanced thermal stability.

Operational stability is one of the main obstacles that may hold back the commercialization of perovskite solar cells (PVSCs). In this paper, we provide a detailed account of the ion migration accelerated PVSC degradation by comparatively studying perovskite materials with two different organic cations (methylammonium (MA+) and formamidinium (FA+)). Using time of flight secondary ion mass spectrometry (TOF-SIMS), we have uncovered the ion migration accelerated degradation of PVSCs at the device level. Not only did mobile iodide (I-) ions from the perovskite layer diffuse out, but Ag atoms/ions from the metal electrode also diffused into the perovskite layer, which resulted in severe device degradation. Besides, we identified I- species in the hole transport material (HTM) layer for even freshly prepared PVSC devices, which was responsible for the degradation of devices kept under inert conditions. This also testifies the existence of ion migration on the device level of PVSCs. Compared with MAPbI3, the ion migration process can slow down in FAPbI3 devices which accounts for a better stability of FAPbI3 devices. This work underscores the impact of organic cation substitution on PVSC degradation and provides solid evidence for mobile ion migration in perovskite materials and the consequent degradation in specific device settings such as the n-i-p type perovskite solar cells. © The Royal Society of Chemistry.

CH3NH3PbBr3 (MAPbBr3)-perovskite solar cells (Br-PSCs) have attracted much attention due to their green-long wavelength transparency and high open-circuit voltage originating from their large bandgap (2.2 eV). However, the efficiency of carbon-based Br-PSCs without organic hole transport materials (HTM) is still low due to the inappropriate quality of MAPbBr3 deposited in a relatively thick porous scaffold. Herein, an amorphous precursor route based on a two-step sequential method is exploited to conformably and seamlessly grow MAPbBr3 in the TiO2 porous scaffold. In the first step, the amorphous Pb–Br precursor containing a large amount of DMF molecules was prepared by lowering the post-treatment temperature to 25 °C, affording full pore filling and smooth surface capping. The conversion to MAPbBr3 in the second step was accelerated by the molecular exchange between DMF and MABr in IPA solution. Moreover, by solvent engineering through the addition of non-polar cyclohexane into the MABr IPA solution, the molecular exchange process was tuned in such a way to separate the nucleation and growth of MAPbBr3 crystals, leading to the preferential [001] orientation with an even surface finishing and the subsequent light absorption enhancement and trap state reduction. Using MAPbBr3 films in carbon-based PSCs has boosted their efficiency to 8.09% (Voc = 1.35 V), a record value for HTM-free Br-PSCs, also comparable to that of the best HTM-based Br-PSCs. Significantly, non-encapsulated devices showed no efficiency decay after storage in dry air (25–30 °C and 10–20% humidity) for 90 days. What is more, the efficiency was retained up to about 90% after storage for 15 days under high heat stress (air, 80 °C and 50–85% humidity).

A new fullerene derivative named C5-NCMA is introduced as an electron transporting material (ETM) to replace the commonly used PCBM in the planar p-i-n perovskite solar cells (PVSCs). Compared with PCBM, this fullerene derivative features a higher hydrophobicity, higher LUMO energy level and higher ability of self-assembly. With the device structure of FTO/NiOx/MAPbI3/ETM/Ag, the C5-NCMA showed power conversion efficiency (PCE) of up to 17.6% with negligible hysteresis, which is higher than PCBM (16.1%). It was found that a higher LUMO energy level was obtained for C5-NCMA compared to PCBM, which favored a higher open-circuit voltage (Voc) in PVSCs with C5-NCMA than PCBM. Besides, the higher electron mobility, photoluminescence (PL) quenching efficiency and quenching rate of C5-NCMA led to more efficient electron transport and charge extraction in the device, thus resulting in a higher fill factor (0.79). Most importantly, the stability of PVSCs to moisture is significantly enhanced for C5-NCMA compared to PCBM due to the hydrophobic nature of C5-NCMA. Thus, we believe that the present work provides an important guide for the further development of ETMs for stable and efficient PVSCs. © 2016 Elsevier Ltd

This work explores the use of poly(3-hexylthiophene) (P3HT) modified carbon nanotubes (CNTs@P3HT) for the cathodes of hole transporter free, mesoscopic perovskite (CH3NH3PbI3) solar cells (PSCs), simultaneously achieving high-performance, high stability and low-cost PSCs. Here the thin P3HT modifier acts as an electron blocker to inhibit electron transfer into CNTs and a hydrophobic polymer binder to tightly cross-link the CNTs together to compact the carbon electrode film and greatly stabilize the solar cell. On the other hand, the presence of CNTs greatly improve the conductivity of P3HT. By optimizing the concentration of the P3HT modifier (2 mg/mL), we have improved the power conversion efficiencies (PCEs) of CNTs@P3HT based PSCs up to 13.43% with an average efficiency of 12.54%, which is much higher than the pure CNTs based PSCs (best PCE 10.59%) and the sandwich-type P3HT/CNTs based PSCs (best PCE 9.50%). In addition, the hysteresis of the CNTs@P3HT based PSCs is remarkably reduced due to the intimate interface between the perovskite and CNTs@P3HT electrodes. Degradation of the CNTs@ P3HT based PSCs is also strongly retarded as compared to cells employing the pure CNTs electrode when exposed to the ambient condition of 20%–40% humidity. © 2016, Higher Education Press and Springer-Verlag Berlin Heidelberg.

Carbon-based hole transport material (HTM)-free perovskite solar cells (PSCs) have attracted intense attention due to their relatively high stability. However, their power conversion efficiency (PCE) is still low, especially for the simplest paintable carbon-based PSCs (C-PSCs), whose performance is greatly limited by poor contact at the perovskite/carbon interface. To enhance interface contact, it is important to fabricate an even-surface perovskite layer in a porous scaffold, which is not usually feasible due to roughness of the crystal precursor. Herein, colloidal engineering is applied to replace the traditional crystal precursor with a colloidal precursor, in which a small amount of dimethyl sulfoxide (DMSO) is added into the conventional PbI2 dimethylformamide (DMF) solution. After deposition, PbI2(DMSO) adduct colloids (which are approximately tens of nanometers in size) are stabilized and dispersed in DMF to form a colloidal film. Compared with PbI2 and PbI2(DMSO) adduct crystal precursors deposited from pure DMF and DMSO solvents, respectively, the PbI2(DMSO) adduct colloidal precursor is highly mobile and flexible, allowing an ultra-even surface to be obtained in a TiO2 porous scaffold. Furthermore, this ultra-even surface is well-maintained after chemical conversion to CH3NH3PbI3 in a CH3NH3I solution. As a result, the contact at the CH3NH3PbI3/carbon interface is significantly enhanced, which largely boosts the fill factor and PCE of C-PSCs. Impressively, the achieved champion PCE of 14.58% is among the highest reported for C-PSCs. © 2016 American Chemical Society.