Chang-Qi Ma

Quantitative operando visualization of the energy band depth profile in solar cells

The energy band alignment in solar cell devices is critically important because it largely governs elementary photovoltaic processes, such as the generation, separation, transport, recombination and collection of charge carriers. Despite the expenditure of considerable effort, the measurement of energy band depth profiles across multiple layers has been extremely challenging, especially for operando devices. Here we present direct visualization of the surface potential depth profile over the cross-sections of operando organic photovoltaic devices using scanning Kelvin probe microscopy. The convolution effect due to finite tip size and cantilever beam crosstalk has previously prohibited quantitative interpretation of scanning Kelvin probe microscopy-measured surface potential depth profiles. We develop a bias voltage-compensation method to address this critical problem and obtain quantitatively accurate measurements of the open-circuit voltage, built-in potential and electrode potential difference.

Acceptor–donor–acceptor-based small molecules with varied crystallinity: processing additive-induced nanofibril in blend film for photovoltaic applications

A series of acceptor-donor-acceptor-based small molecules (SMs) with varied crystallinity were successfully synthesized. The processing additive can induce the SMs to self-organize as nanofibrils with higher crystallinity and controlled scales of nanofibrils, which have significant influence on the photovoltaic performance.

The solvent treatment effect of the PEDOT:PSS anode interlayer in inverted planar perovskite solar cells

Inverted planar perovskite (PVSK) solar cells with a device structure of ITO/PEDOT:PSS/PVSK/PC61BM/Al have emerged as a new generation solar cell owing to their advantages of high power conversion efficiency (PCE), low processing temperature and potential low cost. In this paper, the polar solvent treatment effect of the PEDOT:PSS anode interlayer on PVSK solar cell performance was investigated. The conductivity of the PEDOT:PSS film was found to increase by washing with polar solvents, including H2O, ethanol (EtOH), and a mixture solvent of ethanol and H2O (EtOH:H2O = 8:2 v/v), which was attributed to the removal of the PSS component from the PEDOT:PSS film, leading to a PEDOT-rich surface. However, the PCE of perovskite solar cells decreased from 9.39% for the pristine PEDOT:PSS film based device to 4.21%, 8.35%, 7.13% for the H2O-, EtOH- and EtOH:H2O-treated PEDOT:PSS film based devices, respectively, suggesting that the high conductivity of the PEDOT:PSS film does not ensure a high device performance of the inverted PVSK solar cells. UV-Vis absorption spectra, AFM surface morphology and SEM images of the PVSK films deposited on different PEDOT:PSS surfaces were studied, and the results showed that PEDOT-rich surface is not favorable for the crystal growth of PVSK layer, and consequently leads to poor device performances. This conclusion was further supported by the improved device performance of PVSK solar cells based on a PEDOT:PSS/PSSNa anode buffer layer, where an additional poly(sodium p-styrenesulfonate) (PSSNa) layer was deposited on the PEOT:PSS surface.

Peripherally diketopyrrolopyrrole-functionalized dendritic oligothiophenes – synthesis, molecular structure, properties and applications

Three-dimensional π-conjugated dendrimers are a class of structure defined macromolecules for use in organic electronics. Herein, a new family of dendritic oligothiophenes (DOT-p-DPPs) that are functionalized with the diketopyrrolopyrrole group at the periphery were synthesized by a precise stepwise approach. The chemical structure and the monodisperse nature of these DOT-p-DPPs were confirmed by NMR, MALDI-TOF MS, HR MS, and GPC measurements. UV-vis absorption and fluorescence spectra and cyclic voltammetry data of these compounds were also measured. Small band gaps (∼1.8 eV) and almost identical HOMO/LUMO energy levels (−5.2/−3.5 eV) were measured for these DOT-p-DPPs independent of the molecular size. However, the molecular molar extinction coefficient (e) of DOT-p-DPPs was found to be linearly correlated with the number of terminal DPP units, and a high e of 3.6 × 105 cm−1 L mol−1 was measured for the bigger molecules. These results in combination with theoretical calculation results confirm that the frontier molecular orbitals are mostly localized over the peripheral DPP units. The applications of DOT-p-DPPs in organic solar cells as the electron donor are presented. However, unfavorable nanophase separation and lower DOT-p-DPP content in the blended films led to poor device performance. The two photon absorption cross section of these DPP decorated dendrimers was measured, and high cross section values of over 2000 GM were measured for these dendritic molecules, among which the G1 dendrimer 6T-p-DPP with a high TPA cross section value close to 7000 GM was achieved.

Effect of the π-conjugation length on the properties and photovoltaic performance of A–π–D–π–A type oligothiophenes with a 4,8-bis(thienyl)benzo[1,2-b:4,5-b′]dithiophene core

Summary Benzo[1,2-b:4,5-b′]dithiophene (BDT) is an excellent building block for constructing π-conjugated molecules for the use in organic solar cells. In this paper, four 4,8-bis(5-alkyl-2-thienyl)benzo[1,2-b:4,5-b′]dithiophene (TBDT)-containing A–π–D–π–A-type small molecules (COOP-nHT-TBDT, n = 1, 2, 3, 4), having 2-cyano-3-octyloxy-3-oxo-1-propenyl (COOP) as terminal group and regioregular oligo(3-hexylthiophene) (nHT) as the π-conjugated bridge unit were synthesized. The optical and electrochemical properties of these compounds were systematically investigated. All these four compounds displayed broad absorption bands over 350–600 nm. The optical band gap becomes narrower (from 1.94 to 1.82 eV) and the HOMO energy levels increased (from −5.68 to −5.34 eV) with the increase of the length of the π-conjugated bridge. Organic solar cells using the synthesized compounds as the electron donor and PC61BM as the electron acceptor were fabricated and tested. Results showed that compounds with longer oligothiophene π-bridges have better power conversion efficiency and higher device stability. The device based on the quaterthiophene-bridged compound 4 gave a highest power conversion efficiency of 5.62% with a V OC of 0.93 V, J SC of 9.60 mA·cm−2, and a FF of 0.63.