Yecheng Zhou

Pushing up the efficiency of planar perovskite solar cells to 18.2% with organic small molecules as the electron transport layer

Compared to the traditional-architecture perovskite photovoltaic solar cells (n-i-p type), which use metal oxide as electron transport layers (ETLs) and organic semiconducting materials as hole transport layers, the fabrication of metal-oxide-free, solution-processed inverted perovskite solar cells (PSCs) is more desired because of low-temperatures and all-solution-based applications in future commercial PSC modules. In a typical configuration of inverted PSCs, the widely used ETL compound is the fullerene-based phenyl-C61-butyric acid methyl ester (PCBM), which currently is the best organic ETL material. The cost of this compound is very high, and the morphology and electrical properties are very sensitive to experimental conditions. We here propose a new organic ETL material for the replacement of PCBM in inverted PSCs. We demonstrate metal-oxide-free solution-processed inverted PSCs using the n-type sulfur-containing azaacene 10,14-bis(5-(2-ethylhexyl)thiophen-2-yl)-dipyrido[3,2-a:2′,3′-c][1,2,5]thiadiazolo[3,4-i]phenazine (TDTP) as the ETL with a power conversion efficiency of ∼18.2%, which is higher than that of the corresponding non-sulfur-containing azaacene 10,17-bis((triisopropylsilyl)ethynyl)dipyrido[3,2-a:2′,3′-c]quinoxalino[2,3-i]phenazine (PYPH)-based PSCs (up to 9.5%) or PCBM-based PSCs (up to 17.0%). This superior performance is attributed to the stronger interaction between TDTP and the perovskite surface than that between PYPH and the perovskite surface, which is supported by theoretical calculations. Our results show that easily-accessible simple n-type sulfur-containing small molecules are promising ETL candidates to further propel inverted PSCs to practical applications.

Tunable electronic and magnetic properties of graphene-like ZnO monolayer upon doping and CO adsorption: a first-principles study

Graphene-like zinc oxide monolayer (g-ZnO) is a new class of two-dimensional nanomaterials with unique new properties that is still largely unknown. This work studies the tunability of the electronic and magnetic properties of g-ZnO upon chemical doping (with B, N and C) and CO adsorption by using first-principles calculations. Both electronic and magnetic properties of g-ZnO exhibit strong dependency on its structural change and molecular adsorption. The g-ZnO with oxygen atom substituted by a C or N atom (one atom per supercell) are ferromagnetic (FM) half metal (HM), while that substituted by a B atom is an FM semiconductor. The doped g-ZnO shows strong chemisorption to CO molecule by forming A–CO bond (A = B, N or C), in contrast to the weak physisorption of the intrinsic g-ZnO. Furthermore, CO adsorption converts the N- and C-doped g-ZnO to n-type semiconductor with nonmagnetic (NM) ground states, while B-doped g-ZnO becomes a ferromagnetic half metal (FM-HM). The mechanism for property change has been investigated by analyzing their partial density of states (PDOS) upon different conditions. This study provides insights in the physical properties and chemical reactivity of g-ZnO, which could help in realizing their diverse potentials in electronic and magnetic devices.

Solvent Accommodation: Functionalities Can Be Tailored Through Co-Crystallization Based on 1:1 Coronene-F4TCNQ Charge-Transfer Complex

Because organic donor/acceptor blending systems play critical roles in ambipolar transistors, photovoltaics, and light-emitting transistors, it is highly desirable to precisely tailor the stacking of cocrystals toward different intrinsic structures and physical properties. Here, we demonstrated that the structure-stacking modes and electron-transport behaviors of coronene-F4TCNQ cocrystals (1:1) can be tuned through the solvent accommodation. Our results clearly show that the solvent accommodation not only enlarges the inner mixed packing (...DAD···) distances, leading to the depressed short-contact interactions including the side-by-side and face-by-face intermolecular interactions, but also switches off electron-transport behavior of coronene-F4TCNQ cocrystals (1:1) in ambient atmosphere.

Enhancement of Performance and Mechanism Studies of All-Solution Processed Small-Molecule based Solar Cells with an Inverted Structure.

Both solution-processed polymers and small molecule based solar cells have achieved PCEs over 9% with the conventional device structure. However, for the practical applications of photovoltaic technology, further enhancement of both device performance and stability are urgently required, particularly for the inverted structure devices, since this architecture will probably be most promising for the possible coming commercialization. In this work, we have fabricated both conventional and inverted structure devices using the same small molecular donor/acceptor materials and compared the performance of both device structures, and found that the inverted structure based device gave significantly improved performance, the highest PCE so far for inverted structure based device using small molecules as the donor. Furthermore, the inverted device shows a remarkable stability with almost no obvious degradation after three months. Systematic device physics and charge generation dynamics studies, including optical simulation, light-intensity-dependent current-voltage experiments, photocurrent density-effective voltage analyses, transient absorption measurements, and electrical simulations, indicate that the significantly enhanced performance using inverted device is ascribed to the increasing of Jsc compared to the conventional device, which in turn is mainly attributed to the increased absorption of photons in the active layers, rather than the reduced nongeminate recombination.

Numerical analysis of a hysteresis model in perovskite solar cells

Abstract Previously, we proposed that the polarization and capacitive charge in CH 3 NH 3 PbI 3 screens the external electric field that hinders charge transport. We argue here that this screening effect is in significant part responsible for the power conversion characteristics and hysteresis in CH 3 NH 3 PbI 3 photovoltaic cells. In this paper, we implement capacitive charge and polarization charge into the numerical model that we have developed for perovskite solar cells. Fields induced by these two charges screen the applied hindering field, promote charge transport, and improve solar cell’s performance, especially in solar cells with short diffusion lengths. This is the reason why perovskite solar cells made from simple fabrication methods can achieve high performance. More importantly, with relaxations of capacitive charge and polarization charge, we quantitatively reproduce experimental “anomalous” hysteresis J-V curves. This reveals that both polarization relaxation and ions relaxation could contribute to anomalous hysteresis in perovskite solar cells.

Towards predicting the power conversion efficiencies of organic solar cells from donor and acceptor molecule structures

In this study, we developed a multiscale simulation framework to estimate the power conversion efficiencies of bulk heterojunction organic solar cells based on the molecular structures of the donor and acceptor. Firstly, we proposed a way to estimate the density of states (DOS) of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) in organic thin films based on quantum calculations, and verified the Gaussian-like DOS in the organic semiconductors. Secondly, the electronic couplings in these thin films were calculated. By adding PC71BM molecules, although the donor–donor couplings are not altered significantly, the charge mobility is enhanced via additional donor–acceptor and acceptor–acceptor couplings. Thirdly, random walk simulations were performed to estimate the charge carrier mobilities. Finally, by incorporating the calculated energy levels, mobilities and DOS of these bulk heterojunctions into the numerical model developed, we obtained the working curves and power conversion efficiencies, which are in general consistence with experiment results. This study builds the foundation for the computation of power conversion efficiencies of organic solar cells using fully atomistic simulations.

Polymer-Assisted Single Crystal Engineering of Organic Semiconductors To Alter Electron Transport

A new crystal phase of a naphthalenediimide derivative (α-DPNDI) has been prepared via a facial polymer-assisted method. The stacking pattern of DPNDI can be tailored from the known one-dimensional (1D) ribbon (β phase) to a novel two-dimensional (2D) plate (α phase) through the assistance from polymers. We believe that the presence of polymers during crystal growth is likely to weaken the direct π-π interactions and favor side-to-side C-H-π contacts. Furthermore, β phase architecture shows electron mobility higher than that of the α phase in the single-crystal-based OFET. Theoretical calculations not only confirm that β-DPNDI has an electron transport performance better than that of the α phase but also indicate that the α phase crystal displays 2D transport while the β phase possesses 1D transport. Our results clearly suggest that polymer-assisted crystal engineering should be a promising approach to alter the electronic properties of organic semiconductors.