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

Abstract

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.

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

“…The lowest unoccupied molecular orbital (LUMO) levels were determined using an optical bandgap from the absorption spectra (Figure S1, Supporting Information) and the HOMO levels (Figure S2 and Table S1, Supporting Information). As shown in Figure c, the HOMO and LUMO levels of all studied π‐CP‐HTMs were appropriate to extract holes and block electrons from nCQD‐ink photoactive layers . The EHT‐BDT based π‐CP‐HTMs displayed deeper HOMO levels than did EHO‐BDT based π‐CP (PTB7), while PBDTTPD‐HT exhibited the lowest HOMO level (−5.03 eV), which might be beneficial for hole extraction.…”

“…A certified PCE of 11.04% was obtained (Figure S6, Supporting Information), which agrees well with the measurement results in our lab. Notably, the PCE of the PBDTTPD‐HT device (11.53%) was the highest among the CQDSCs using organic HTMs, and even higher than that of the reported record high SSE‐free CQDSC using pCQD‐HTM (10.91%) . Because it is well known that the optimal anode for pCQD‐HTM is Au, we compared the performance of devices with a conventional pCQD‐HTM (EDT‐PbS by SSE) using either MoO x /Ag or Au as the anode (Figure S7, Supporting Information).…”

“…To further evaluate the interfacial charge extraction, we performed C p – V measurement at various illumination intensities (Figure d–f) . The charge accumulation at the CQD/HTM interfaces can be elucidated by monitoring the shift of the peak voltage ( V peak ) value at various illumination intensities.…”

“…Device and Optical Simulation : SCAPS software (3.3.07 version) was used to simulate the CQDSC performance parameter under AM 1.5G 100 mW cm −2 illumination with respect to HOMO level and hole mobility . The parameters used in this simulation was obtained from the previous report, with slight modification, and are summarized in Table S3 of the Supporting Information.…”

Spatial Collection in Colloidal Quantum Dot Solar Cells

“…As opposed to analytical methods, regularization‐based methods such as the one presented herein work even when single‐pass light absorption is far from complete, a situation common to most perovskite, organics, and CQD devices. In addition, the Gaussian‐noise regularization scheme allows one to resolve sharp features in SCE profiles arising from surface recombination, interfacial defects, or heterojunctions, making it promising in the study of the degradation of perovskite active layers near their electron‐ and hole‐transport layers or CQD‐organic hybrid devices, for instance.…”