Colloidal-quantum-dot (CQD) photovoltaic devices are promising candidates for low-cost power sources owing to their low-temperature solution processability and bandgap tunability. A power conversion efficiency (PCE) of >10% is achieved for these devices; however, there are several remaining obstacles to their commercialization, including their high energy loss due to surface trap states and the complexity of the multiple-step CQD-layer-deposition process. Herein, high-efficiency photovoltaic devices prepared with CQD-ink using a phase-transfer-exchange (PTE) method are reported. Using CQD-ink, the fabrication of active layers by single-step coating and the suppression of surface trap states are achieved simultaneously. The CQD-ink photovoltaic devices achieve much higher PCEs (10.15% with a certified PCE of 9.61%) than the control devices (7.85%) owing to improved charge drift and diffusion. Notably, the CQD-ink devices show much lower energy loss than other reported high-efficiency CQD devices. This result reveals that the PTE method is an effective strategy for controlling trap states in CQDs.

Solid-state-ligand-exchange free high-efficiency colloidal quantum dot solar cells were developed by direct coating of n-type and p-type quantum dot inks.

IntroductionColloidal quantum dots (CQDs) offer advantages over conventional bulk inorganic semiconductors such as solution Low-temperature solution-processed high-effi ciency 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 modifi cation 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 benefi ts 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 fi gures-of-merit of the devices are improved as a result of the enhanced built-in potential (electric fi eld) and the reduced interfacial charge recombination at the heterojunction. The benefi ts 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 effi ciency 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.

IntroductionColloidal quantum dot (CQD) based photovoltaic devices (CQDPVs) have emerged as promising next-generation solar cells owing to low-cost solution processibility at low temperature, easy bandgap tunability into the near infrared (NIR, λ > 800 nm) regime, and multiple exciton generation. [1,2] The power conversion efficiency (PCE) of CQDPVs has improved 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.

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