Zhaowei Wang

Room-temperature mixed-solvent-vapor annealing for high performance perovskite solar cells

In this paper, we introduce a room-temperature mixed-solvent-vapor annealing (rtMSVA) method to fabricate high performance perovskite solar cells (pero-SCs) based on MAPbI3−xClx without the need for thermal annealing (TA). An ultra-smooth perovskite thin-film with high crystallinity was obtained by the DMF/CB mixed-solvent (1 : 20, v/v) vapor annealing at room-temperature without TA and the power conversion efficiency (PCE) of the pero-SCs reached 16.4%. More importantly, the reproducibility of the PCEs is quite good among 40 different devices. Furthermore, large active area pero-SCs were fabricated with the rtMSVA method. The PCEs of the pero-SCs based on ITO and flexible PET/Ag mesh electrodes with an active area of 1.21 cm2 reached 11.01% and 7.5%, respectively. We anticipate that rtMSVA would very possibly become a promising crystallization method for the fabrication of large area pero-SCs in the near future.

Facilitating Electron Transportation in Perovskite Solar Cells via Water-Soluble Fullerenol Interlayers

TiO2 is widely used in perovskite solar cells (Pero-SCs), but its low electrical conductivity remains a drawback for application in electron transport layer (ETL). To overcome this problem, an easily accessible hydroxylated fullerene, fullerenol, was employed herein as ETL modified on ITO in n-i-p type (ITO as cathode) Pero-SCs for the first time. The results showed that the insertion of a single layer of fullerenol between perovskite and TiO2 dramatically facilitates the charge transportation and decreases the interfacial resistance. As a consequence, the device performance was greatly improved, and a higher power conversion efficiency of 14.69% was achieved, which is ∼17.5% enhancement compared with that (12.50%) of the control device without the fullerenol interlayer. This work provides a new candidate of interfacial engineering for facilitating the electron transportation in Pero-SCs.

All-small-molecule organic solar cells based on an electron donor incorporating binary electron-deficient units

In this work, solution-processed organic solar cells with conjugated small molecules both as electron donors and electron acceptors were studied, where the influence of the chemical structures of the donor and acceptor on the device performance was systematically investigated. A small molecular donor incorporating binary electron-deficient units, diketopyrrolopyrrole and pentacyclic aromatic bislactam, was synthesized to provide a low band gap of 1.65 eV and low-lying energy levels. Three molecules, from a fullerene derivative to non-fullerene perylene bisimide-based acceptors, were selected as electron acceptors to construct organic solar cells. The results showed that fullerene-based solar cells provided power conversion efficiencies (PCEs) of up to 4.8%, while the non-fullerene solar cells also exhibited promising PCEs of 2.4% and 3.5%, with a photoresponse of up to 750 nm. Further analysis of the bulk-heterojunction systems between donors and acceptors revealed that the relatively low carrier mobilities of the non-fullerene acceptors and the large phase separations are mainly responsible for the less efficient solar cells. Our results demonstrate that molecules containing several electron-deficient units can effectively reduce the band gap of small molecules, and thus offer great potential for realizing high performance fullerene and non-fullerene solar cells.

Water-Soluble 2D Transition Metal Dichalcogenides as the Hole-Transport Layer for Highly Efficient and Stable p–i–n Perovskite Solar Cells

As a hole-transport layer (HTL) material, poly(3,4-ethylenedioxythiophene):polystyrene-sulfonate (PEDOT:PSS) was often criticized for its intrinsic acidity and hygroscopic effect that would in the long run affect the stability of perovskite solar cells (Pero-SCs). As alternatives, herein water-soluble two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS2 and WS2 were used as HTLs in p-i-n Pero-SCs. It was found that the content of 1T phase in 2D TMDs HTLs is centrally important to the power conversion efficiencies (PCEs) of Pero-SCs, and the 1T-rich TMDs (as achieved from exfoliation and without postheating) lead to much higher PCEs. More importantly, as PEDOT:PSS was replaced by 2D TMDs, both the PCEs and stability of Pero-SCs were significantly improved. The highest PCEs of 14.35 and 15.00% were obtained for the Pero-SCs with MoS2 and WS2, respectively, whereas the Pero-SCs with PEDOT:PSS showed a highest PCE of only 12.44%. These are up to date the highest PCEs using 2D TMDs as HTLs. After being stored in a glovebox for 56 days, PCEs of the Pero-SCs using MoS2 and WS2 HTLs remained 78 and 72%, respectively, whereas the PCEs of Pero-SCs with PEDOT:PSS almost dropped to 0 over 35 days. This study demonstrates that water-soluble 2D TMDs have great potential for application as new generation of HTLs aiming at high performance and long-term stable Pero-SCs.

All polymer solar cells with diketopyrrolopyrrole-polymers as electron donor and a naphthalenediimide-polymer as electron acceptor

Four typical diketopyrrolopyrrole (DPP)-based conjugated polymers were used as electron donors in all-polymer solar cells (PSCs) with a naphthalenediimide-based polymer N2200 as the electron acceptor. The four DPP polymers have near-infrared absorption spectra up to 1000 nm and suitable energy levels for charge separation from donor to acceptor. DPP polymer : N2200 cells were found to have high open circuit voltages in comparison to fullerene-based solar cells but with low short circuit current densities and fill factors, so that the power conversion efficiencies of these cells were relatively low (0.45–1.7%). These blends relatively had balanced but low hole and electron mobilities from space charge limit current measurements, small surface roughness, and highly quenched photoluminescence (PL) from steady-state PL. These studies show that the low photocurrent and performance arise from the miscibility of the DPP and N2200 polymers, which enhances the charge recombination. The finding was further confirmed by grazing incidence X-ray diffraction and resonant soft X-ray scattering. All the PSCs based on DPP polymers were investigated, opening further studies based on these systems due to the broad absorption, high carrier mobilities and good crystalline properties of DPP polymers.

Comprehensive Study of Sol–Gel versus Hydrolysis–Condensation Methods To Prepare ZnO Films: Electron Transport Layers in Perovskite Solar Cells

Owing to the high charge mobility and low processing temperature, ZnO is regarded as an ideal candidate for electron transport layer (ETL) material in thin-film solar cells. For the film preparation, the presently dominated sol-gel (SG) and hydrolysis-condensation (HC) methods show great potential; however, the effect of these two methods on the performance of the resulting devices has not been investigated in the same frame. In this study, the ZnO films made through SG and HC methods were applied in perovskite solar cells (Pero-SCs), and the performances of corresponding devices were compared under parallel conditions. We found that the surface morphologies and the conductivities of the films prepared by SG and HC methods showed great differences. The HC-ZnO films with higher conductivity led to relatively higher device performance, and the best power conversion efficiencie (PCE) of 12.9% was obtained; meanwhile, for Pero-SCs based on SG-ZnO, the best PCE achieved was 10.9%. The better device performance of Pero-SCs based on HC-ZnO should be attributed to the better charge extraction and transportation ability of HC-ZnO film. Moreover, to further enhance the performance of Pero-SCs, a thin layer of pristine C60 was introduced between HC-ZnO and perovskite layers. By doing so, the quality of perovskite films was improved, and the PCE was elevated to 14.1%. The preparation of HC-ZnO film involves relatively lower-temperature (maximum 100 °C) processing; the films showed better charge extraction and transportation properties and can be a more promising ETL material in Pero-SCs.

Towards a full understanding of regioisomer effects of indene-C60 bisadduct acceptors in bulk heterojunction polymer solar cells

Indene-C60 bisadduct (IC60BA), which can offer a significantly higher open-circuit voltage (Voc) than monoadducts, has become the research focus as electron acceptor materials in polymer solar cells (PSCs) in recent years. However, despite its popularity, IC60BA have always been applied in PSCs as mixture of several regioisomers and the nature of this mixture has never been fully investigated and understood. Herein, for the first time, 12 major regioisomers of IC60BA were isolated and a full investigation was carried out with respect to their structure, abundance, solubility and their corresponding photovoltaic performance. The results show that the PSCs based on these regioisomeric structures present very diverse PCE and their photovoltaic performance was dramatically affected not only by the relative indene positions but also by the steric orientation of the two indene groups. Electrochemical studies further revealed that the effect of energetic disorder inside the IC60BA regioisomers on their photovoltaic performance is insignificant when applied in PSCs. However, the steric structures and solubility of the regioisomers were found to have significant impact on the morphology and bulk properties of the active layer of PSCs, which give rise to very different PCE of devices based on IC60BA regioisomers with different structures.

Tuning Surface Energy of Conjugated Polymers via Fluorine Substitution of Side Alkyl Chains: Influence on Phase Separation of Thin Films and Performance of Polymer Solar Cells

Different contents of fluorine in side alkyl chains were incorporated into three conjugated polymers (namely, PBDTTT-f13, PBDTTT-f9, and PBDTTT-f5) whose backbones consist of benzodithiophene donors and thienothiophene acceptors. These three fluorinated polymers, in comparison with the well-known analogue PTB7-Th, show comparable energy levels and optical band gaps. However, the fluorination of side alkyl chains significantly changed the surface energy of bulk materials, which leads to distinctly different self-assembly behaviors and phase separations as being mixed with PC71BM. The increased mismatch in surface energies between the polymer and PC71BM causes larger scale phase domains, which makes a sound explanation for the difference in their photovoltaic properties.

Chemical Modification of n-Type-Material Naphthalene Diimide on ITO for Efficient and Stable Inverted Polymer Solar Cells

To provide orthogonal solvent processable surface modification and improve the device stability of bulk-heterojunction polymer solar cells (PSCs), n-type semiconducting material naphthalene diimide (NDI) was chemically introduced onto the ITO surface as a cathode interlayer (CIL) using 3-bromopropyltrimethoxysilane (BrTMS) as a coupling agent. After modification, the work function of ITO can be decreased from 4.70 to 4.23 eV. The modified ITO cathode was applied in inverted PSCs based on PTB7-Th:PC71BM. With the CIL modification, a champion power conversion efficiency (PCE) of 5.87% was achieved, showing a dramatic improvement compared to that of devices (PCE = 3.58%) without CIL. More importantly, with these chemical bonded interlayers, the stability of inverted PSCs was greatly enhanced. The improved PCE and stability can be attributed to the increased open-circuit voltage and the formation of robust chemical bonds in NDI-TMS films, respectively. This study demonstrated that chemical modification of ITO with n-type semiconducting materials provides an avenue for not only solving the solvent orthogonal problem but also improving the device performance in terms of the PCE and the stability.

Semi-transparent perovskite solar cells: unveiling the trade-off between transparency and efficiency

Semi-transparent perovskite solar cells (Pero-SCs) are realized by tuning the band gap of the perovskite to resolve the trade-off between the transparency and efficiency of the photo-absorber. We synthesized wide-bandgap MAPbI3−xBrx perovskite, and the transparency and efficiency of the corresponding semi-transparent Pero-SCs were investigated systematically by varying the I : Br ratio and thickness of the perovskite film. Increasing Br content widened the bandgap of perovskite (i.e., blue shift of the absorption edge), and led to an increase in the average visible transmittance (AVT). This strategy allowed for high AVTs, and concomitantly achieved high power conversion efficiencies. Meanwhile, increasing the Br content could facilitate formation of perovskite films with large grains that were highly crystallized. Compared with the narrow-bandgap perovskite, the wide-bandgap perovskite showed advantages for obtaining semi-transparent Pero-SCs with thick perovskite films (>200 nm) and high (20%) transparency.