Zhiqiang Guan

Decomposition of Organometal Halide Perovskite Films on Zinc Oxide Nanoparticles

Solution processed zinc oxide (ZnO) nanoparticles (NPs) with excellent electron transport properties and a low-temperature process is a viable candidate to replace titanium dioxide (TiO2) as electron transport layer to develop high-efficiency perovskite solar cells on flexible substrates. However, the number of reported high-performance perovskite solar cells using ZnO-NPs is still limited. Here we report a detailed investigation on the chemistry and crystal growth of CH3NH3PbI3 perovskite on ZnO-NP thin films. We find that the perovskite films would severely decompose into PbI2 upon thermal annealing on the bare ZnO-NP surface. X-ray photoelectron spectroscopy (XPS) results show that the hydroxide groups on the ZnO-NP surface accelerate the decomposition of the perovskite films. To reduce the decomposition, we introduce a buffer layer in between the ZnO-NPs and perovskite layers. We find that a commonly used buffer layer with small molecule [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) can slow down but cannot completely avoid the decomposition. On the other hand, a polymeric buffer layer using poly(ethylenimine) (PEI) can effectively separate the ZnO-NPs and perovskite, which allows larger crystal formation with thermal annealing. The power conversion efficiencies of perovskite photovoltaic cells are significantly increased from 6.4% to 10.2% by replacing PC61BM with PEI as the buffer layer.

Spectroscopic study on the impact of methylammonium iodide loading time on the electronic properties in perovskite thin films

Solution processed metal–organic halide perovskite photovoltaic devices have recently drawn tremendous attention due to their simplicity of fabrication and high efficiency. Despite numerous reports on optimizing perovskite films with different fabrication approaches, there is limited understanding on the correlation between sensitive processing conditions and the microstructural and electronic properties of perovskite films. Here we combine several opto-electrical spectroscopy techniques to investigate the methylammonium iodide (MAI) loading time effect on the doping density profile and uncoordinated ions in resulting CH3NH3PbI3 perovskite thin films. We find that even in a very short period of different loading times within two minutes, there is a significant impact on the device power conversion efficiency (PCE) from 2% to over 15%. It is found that the doping density profile is inhomogeneous across the perovskite film with too short MAI loading time, resulting in an S-shape in the current density–voltage (J–V) characteristics. On the other hand, devices with too long loading time have excess uncoordinated ions attributed to the J–V hysteresis. By using combined spectroscopy techniques to pinpoint the electronic properties in perovskite films, this work would shed light on the understanding of the controversial origins of the reported S-shape and hysteresis in perovskite photovoltaic cells.

Graphene oxide as an efficient hole-transporting material for high-performance perovskite solar cells with enhanced stability

In recently developed high-efficiency metal organometal halide perovskite solar cells (PVSCs), electron and hole transporting materials have shown key roles in determining the growth of perovskite crystals, as well as the performance and stability of the device. However, interlayer materials which can facilitate both high efficiency and stability at low cost are still limited. Here, we demonstrate that, by controlling the thickness of solution-processed graphene oxide (GO), one can achieve a balance of high work function and conductivity. Using GO with the optimized thickness as a hole-transporting material (HTM) in PVSCs, a high power conversion efficiency (PCE) of 16.5% with no hysteresis has been achieved with excellent light-soaking photocurrent stability in comparison with a commonly used organic-based HTM. Under high humidity and continuous light soaking, the encapsulated perovskite devices retained >80% of their initial efficiency for >2000 h. Detailed studies on the GO binding energy, charge transfer efficiency with perovskite, and crystal morphology shed light on the origin of the observed improvement in photovoltaic performance. Benefiting from the merits of low temperature, solution processability and low cost, the proposed GO fabrication methods could aid scalable production of PVSCs with high PCE and excellent stability.

The detrimental effect of excess mobile ions in planar CH3NH3PbI3 perovskite solar cells

The origin of the impact of mobile ions in perovskite solar cells (PVSCs) has recently become a hot topic of debate. Here, we investigate systematically the structural effect and various recombination pathways in PVSCs with different ion concentrations. By probing the transient ionic current in PVSCs, we extract mobile ion concentrations in a range of 1016 cm−3 to 1017 cm−3 depending on the processing conditions during a two-step process. The PVSC with the lowest ion concentration has both the highest efficiency over 15% and shelf-life over 1300 hours. Interestingly, in contrast to the commonly adopted models in the literature, we find that the crystal size and the bimolecular and trap-assisted recombination are not responsible for the large difference in photovoltaic performance. Instead, by using transient photocurrent and steady-state photoluminescence approaches, we find that the large reduction of short-circuit current (Jsc) in mobile ion populated devices is ascribed to the slow decay in photocurrent and the increasing amount of non-radiative recombination. In addition, we also find that the excess mobile ions trigger the deformation of perovskite to PbI2, which severely reduces the device lifetime. The results provide valuable information on the understanding of the role of excess mobile ions in the degradation mechanism of PVSCs.

Probing the Energy Level Alignment and the Correlation with Open-Circuit Voltage in Solution-Processed Polymeric Bulk Heterojunction Photovoltaic Devices.

Energy level alignment at the organic donor and acceptor interface is a key to determine the photovoltaic performance in organic solar cells, but direct probing of such energy alignment is still challenging especially for solution-processed bulk heterojunction (BHJ) thin films. Here we report a systematic investigation on probing the energy level alignment with different approaches in five commonly used polymer:[6,6]-phenyl-C71-butyric acid methyl ester (PCBM) BHJ systems. We find that by tuning the weight ratio of polymer to PCBM the electronic features from both polymer and PCBM can be obtained by photoemission spectroscopy. Using this approach, we find that some of the BHJ blends simply follow vacuum level alignment, but others show strong energy level shifting as a result of Fermi level pinning. Independently, by measuring the temperature-dependent open-circuit voltage (VOC), we find that the effective energy gap (Eeff), the energy difference between the highest occupied molecular orbital of the polymer donor (EHOMO-D) and lowest unoccupied molecular orbital of the PCBM acceptor (ELUMO-A), obtained by photoemission spectroscopy in all polymer:PCBM blends has an excellent agreement with the extrapolated VOC at 0 K. Consequently, the photovoltage loss of various organic BHJ photovoltaic devices at room temperature is in a range of 0.3-0.6 V. It is believed that the demonstrated direct measurement approach of the energy level alignment in solution-processed organic BHJ will bring deeper insight into the origin of the VOC and the corresponding photovoltage loss mechanism in organic photovoltaic cells.

Evidence on Enhanced Exciton Polarizability in Donor/Acceptor Bulk Heterojunction Organic Photovoltaics

Using electroabsorption spectroscopy, we explore the polarizability of Frenkel excitons in both pristine donor and D/A blend films. We observe for the first time that the polarizability of excitonic states in pristine donors can be dramatically increased by adding an n-type acceptor. By investigating the dielectric effect in different organic semiconductor systems, we find that the polarizability of Frenkel excitons has direct correlation with the measured dielectric constant of the bulk heterojunction thin films. Our results disclose the difference in the nature of Frenkel excitons in pristine donor and D/A blend systems, revealing an important role of excitonic states in charge separation process of organic photovoltaic devices.