Improved Processability and Efficiency of Colloidal Quantum Dot Solar Cells Based on Organic Hole Transport Layers

High‐efficiency solid‐state‐ligand‐exchange (SSE) step‐free colloidal quantum dot photovoltaic (CQDPV) devices are developed by employing CQD ink based active layers and organic (Polythieno[3,4‐b]‐thiophene‐co‐benzodithiophene (PTB7) and poly(3‐hexylthiophene) (P3HT)) based hole transport layers (HTLs). The device using PTB7 as an HTL exhibits superior performance to that using the current leading organic HTL, P3HT, because of favorable energy levels, higher hole mobility, and facilitated interfacial charge transfer. The PTB7 based device achieves power conversion efficiency (PCE) of 9.60%, which is the highest among reported CQDPVs using organic HTLs. This result is also comparable to the PCE of an optimized device based on a thiol‐exchanged p‐type CQD, the current‐state‐of‐the‐art HTL. From the viewpoint of device processing, the fabrication of CQDPVs is achieved by direct single‐coating of CQD active layers and organic HTLs at low temperature without SSE steps. The experimental results and device simulation results in this work suggest that further engineering of organic HTL materials can open new doors to improve the performance and processing of CQDPVs.     

Molecular Engineering in Hole Transport π‐Conjugated Polymers to Enable High Efficiency Colloidal Quantum Dot Solar Cells

Organic p‐type materials are potential candidates as solution processable hole transport materials (HTMs) for colloidal quantum dot solar cells (CQDSCs) because of their good hole accepting/electron blocking characteristics and synthetic versatility. However, organic HTMs have still demonstrated inferior performance compared to conventional p‐type CQD HTMs. In this work, organic π‐conjugated polymer (π‐CP) based HTMs, which can achieve performance superior to that of state‐of‐the‐art HTM, p‐type CQDs, are developed. The molecular engineering of the π‐CPs alters their optoelectronic properties, and the charge generation and collection in CQDSCs using them are substantially improved. A device using PBDTTPD‐HT achieves power conversion efficiency (PCE) of 11.53% with decent air‐storage stability. This is the highest reported PCE among CQDSCs using organic HTMs, and even higher than the reported best solid‐state ligand exchange‐free CQDSC using pCQD‐HTM. From the viewpoint of device processing, device fabrication does not require any solid‐state ligand exchange step or layer‐by‐layer deposition process, which is favorable for exploiting commercial processing techniques.     

Efficient Hybrid Tandem Solar Cells Based on Optical Reinforcement of Colloidal Quantum Dots with Organic Bulk Heterojunctions

While colloidal quantum dot photovoltaic devices (CQDPVs) can achieve a power conversion efficiency (PCE) of ≈12%, their insufficient optical absorption in the near‐infrared (NIR) regime impairs efficient utilization of the full spectrum of visible light. Here, high‐efficiency, solution‐processed, hybrid series, tandem photovoltaic devices are developed featuring CQDs and organic bulk heterojunction (BHJ) photoactive materials for front‐ and back‐cells, respectively. The organic BHJ back‐cell efficiently harvests the transmitted NIR photons from the CQD front‐cell, which reinforces the photon‐to‐current conversion at 350–1000 nm wavelengths. Optimizing the short‐circuit current density balance of each sub‐cell and creating a near ideal series connection using an intermediate layer achieve a PCE (12.82%) that is superior to that of each single‐junction device (11.17% and 11.02% for the CQD and organic BHJ device, respectively). Notably, the PCE of the hybrid tandem device is the highest among the reported CQDPVs, including single‐junction devices and tandem devices. The hybrid tandem device also exhibits almost negligible degradation after air storage for 3 months. This study suggests a potential route to improve the performance of CQDPVs by proper hybridization with NIR‐absorbing photoactive materials.     

Low‐Temperature‐Processed 9% Colloidal Quantum Dot Photovoltaic Devices through Interfacial Management of p–n Heterojunction

Low‐temperature solution‐processed high‐efficiency colloidal quantum dot (CQD) photovoltaic devices are developed by improving the interfacial properties of p–n heterojunctions. A unique conjugated polyelectrolyte, WPF‐6‐oxy‐F, is used as an interface modification layer for ZnO/PbS‐CQD heterojunctions. With the insertion of this interlayer, the device performance is dramatically improved. The origins of this improvement are determined and it is found that the multifunctionality of the WPF‐6‐oxy‐F interlayer offers the following essential benefits for the improved CQD/ZnO junctions: (i) the dipole induced by the ionic substituents enhances the quasi‐Fermi level separation at the heterojunction through favorable energy band‐bending, (ii) the ethylene oxide groups containing side chains can effectively passivate the interfacial defect sites of the heterojunction, and (iii) these effects occur without deterioration in the intrinsic depletion region or the series resistance of the device. All of the figures‐of‐merit of the devices are improved as a result of the enhanced built‐in potential (electric field) and the reduced interfacial charge recombination at the heterojunction. The benefits due to the WPF‐6‐oxy‐F interlayer are generally applicable to various types of PbS/ZnO heterojunctions. Finally, CQD photovoltaic devices with a power conversion efficiency of 9% are achievable, even by a solution process at room temperature in an air atmosphere. The work suggests a useful strategy to improve the interfacial properties of p–n heterojunctions by using polymeric interlayers.     

The relative roles of N fixation,fertilizer, crop residues and soil in supplying N in multiple cropping systems in a humid,tropical upland cropping system

Resistance to root growth in the original flexible-sided cells used by Goss (1977) to study the effect of mechanical impedance on root growth was estimated by two methods. Firstly, the resistance to pushing a 2-mm diameter 30° semiangle metal probe into the cells containing either saturated ballotini, or air-dry or saturated sand was measured. Secondly, the pressure required to inflate a 3-mm diameter thin walled latex tube was measured in the cells. Penetrometer resistance was more than forty times greater than the pressure applied externally to the cell. The pressure required for tube inflation was highly dependent on the experimental procedure followed, but the pressure required to inflate an initially deflated tube exceeded the externally applied pressure by a factor of between 5 and 9 times. These findings suggest that it is necessary to reinterpret the results of many root growth experiments designed to quantify the effects of mechanical impedance on root growth rates and performed in similar cells, and have implications for the future conduct and interpretation of such experiments.