Insight into Air Degradation of Inverted Perovskite Solar Cells with Different Cathode Interlayers.

Air stability is a big challenge for inverted perovskite solar cells (IPVSCs). We focus on effect of a cathode interlayer (BCP or TOASiW12) on air degradation of IPVSCs with an Al or Ag cathode. Combined measurements have been carried out to check the changes of the device electrical performance with exposure to air. Our results demonstrated that the IPVSCs with BCP/Al suffered an overall deterioration in terms of dissociation of excitons, transport, and extraction of charge carriers, which was accompanied by improved trap density and serious trap-induced recombination when exposed to air. Instead, all the electrical characteristics of the IPVSCs with TOASiW12/Al, BCP/Ag and TOASiW12/Ag remained stable or slightly reduced after exposed to air over 2 days. This work provides new insight into the air aging of IPVSCs and facilitates the development of CIL materials for cost-effective IPVSCs.

Tuning an Electrode Work Function Using Organometallic Complexes in Inverted Perovskite Solar Cells.

Low-work-function (WF) metals (including silver (Ag), aluminum (Al), and copper (Cu)) used as external cathodes in inverted perovskite solar cells (PSCs) encounter oxidation caused by air exposure and halogen-diffusion-induced corrosion, which threaten the long-term stability of the device. The cathode interlayer (CIL) has shown promise in reducing the metal WF and thus boosting the device power conversion efficiency (PCE). However, it remains a challenge for current CIL materials to enable high-WF metals (e.g., Au) to be used as cathodes to achieve PSCs with a superior PCE and long-term stability. Here, we use a series of synthesized (carbolong-derived) organometallic complexes as CILs to tune the electrode WF in inverted PSCs. Density functional theory calculations and surface characterizations show that the organometallic complexes that contain anions and cations are prone to form anion-cation dipoles on the metal surface, hence drastically reducing the metal's WF. Photovoltaic devices based on a Ag cathode, which was modified with these organometallic complexes, received a boosted PCE up to 21.29% and a remarkable fill factor that reached 83.52%, which are attributed to the dipole-enhanced carrier transport. The environmental stability of PSCs was further improved after employing Au as a cathode with these organometallic complexes, and the modified devices exhibited no efficiency loss after 4080 h storage measurements.

Efficient Inverted Perovskite Solar Cells Enabled by Dopant-Free Hole-Transporting Materials Based on Dibenzofulvene-Bridged Indacenodithiophene Core Attaching Varying Alkyl Chains.

Inspired by the structural advantages of spiro-OMeTAD, which is the most commonly used hole-transporting material (HTM), two rationally designed HTMs with butterfly-shaped triarylamine groups based on dibenzofulvene-bridged indacenodithiophene (IDT) core (attaching hexyl and octyl chains) have been synthesized, namely, IT-C6 and IT-C8, respectively. Shorter alkyl-chain-based IT-C6 exhibits a marked increase in glass-transition temperature (Tg) of 105 °C, whereas IT-C8 shows a Tg of 95 °C. Moreover, it is demonstrated that IT-C6 exhibits a higher hole-transporting mobility, more suitable band energy alignment, better interfacial contact, and passivation effect. The inverted devices of employed HTM based on IT-C6 obtained a champion PCE of 18.34% with a remarkable fill factor (FF) of 82.32%, whereas the IT-C8-based device delivered an inferior PCE of 16.94% with an FF up to 81.20%. Both HTMs embodied inverted devices present high FF values greater than 81%, which are among the highest reported values of small molecular HTM-based PSCs. This work reveals that cutting off the symmetrical spiro-core and subsequently combining IDT (attaching tailored alkyl chains) with the spiro-linkage fluorine to construct the orthogonal molecular conformation is a significant principle for the design of promising dopant-free HTMs.

Revealing working mechanisms of PFN as a cathode interlayer in conventional and inverted polymer solar cells.

Energy level alignments at the PC71BM/PFN/Ag interface in conventional polymer solar cells (c-PSCs) and the ITO/PFN/PC71BM interface in inverted polymer solar cells (i-PSCs) are systematically investigated via ultraviolet photoelectron spectroscopy and by using the integer charge transfer (ICT) model. The findings demonstrate that PFN as a cathode interlayer is able to effectively reduce the electron extraction barriers from 0.72 eV to 0.38 eV for the c-PSCs and from 0.58 eV to 0.36 eV for the i-PSCs, respectively. In the c-PSCs, the final modified electron extraction barrier matches the predicted value (∼0.4 eV) using the ICT model. In the i-PSCs, there exists an intermixing layer of PFN and the active layer above PFN because some PFN is dissolved by the organic solvent in the active layer solution, resulting in a special energy level alignment at the PFN/PC71BM interface. ITO/PFN (2 nm)/PC71BM (20 nm) in the i-PSCs actually forms such an interface as ITO/PFN/PFN:PC71BM with an energy level alignment like Al/LiF/PC71BM/PFN (0.65 nm), which rationalizes a higher short circuit and fill factor in the i-PSCs than c-PSCs. Finally, a general model to simulate the intermixing layer between the organic cathode interlayer and the active layer has been proposed to describe the energy level alignment of the complicated interfaces in the i-PSCs.

Alcohol-Soluble Isoindigo Derivative IIDTh-NSB as a Novel Modifier of ZnO in Inverted Polymer Solar Cells.

Alcohol-soluble isoindigo derivative with thiophene groups and sulfobetaine zwitterions, IIDTh-NSB was applied as a novel modifier of ZnO in inverted polymer solar cells (i-PSCs). When IIDTh-NSB (0.2 mg/mL) was spin-coated on ZnO as an electron transport layer (ETL), power conversion efficiency (PCE) of the PTB7:PC71BM based i-PSCs reached 8.88%, which is a 20% improvement of that of 7.40% for the device with the ZnO-only ETL. If ZnO was doped by IIDTh-NSB of 1.0 wt %, the PCE of 8.50% could be achieved in the i-PSCs. Combined measurements of capacitance-voltage characteristics, carrier mobility, and photocurrent density-effective voltage characteristics revealed that incorporating IIDTh-NSB as the modifier of ZnO by coating or doping enhanced the built-in potential, charge carrier density and mobility, exciton dissociation, and charge carrier extraction in the i-PSCs because of the improved interfacial contact between the photoactive layer and ZnO as shown in water contact angle measurements and atomic force microscopy images. Finally, impedance spectroscopy investigation provided strong lines of evidence that incorporating IIDTh-NSB as the modifier of ZnO led to the great enhancement in short-circuit current density and fill factor. Furthermore, all the devices with IIDTh-NSB as a modifier of ZnO presented better stability than the device with ZnO-only. These findings suggest that IIDTh-NSB is an effective and competitive material for modification of ZnO in the i-PSCs.

High Performance Small-Molecule Cathode Interlayer Materials with D-A-D Conjugated Central Skeletons and Side Flexible Alcohol/Water-Soluble Groups for Polymer Solar Cells.

A new class of organic cathode interfacial layer (CIL) materials based on isoindigo derivatives (IID) substituted with pyridinium or sulfonate zwitterion groups were designed, synthesized, and applied in polymer solar cells (PSCs) with PTB7:PC71BM (PTB7: polythieno[3,4-b]-thiophene-co-benzodithiophene and PC71BM: [6,6]-phenyl C71-butyric acidmethyl ester) as an active layer. Compared with the control device, PSCs with an IID-based CIL show simultaneous enhancement of open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF). Systematic optimizations of the central conjugated core and side flexible alcohol-soluble groups demonstrated that isoindigo-based CIL material with thiophene and sulfonate zwitterion substituent groups can efficiently enhance the PSC performance. The highest power conversion efficiency (PCE) of 9.12%, which is 1.75 times that of the control device without CIL, was achieved for the PSC having an isoindigo-based CIL. For the PSCs with an isoindigo-based CIL, the molecule-dependent performance property studies revealed that the central conjugated core with D-A-D characteristics and the side chains with sulfonate zwitterions groups represents an efficient strategy for constructing high performance CILs. Our study results may open a new avenue toward high performance PSCs.

Aqueous Solution Processed Photoconductive Cathode Interlayer for High Performance Polymer Solar Cells with Thick Interlayer and Thick Active Layer.

An aqueous-solution-processed photoconductive cathode interlayer is developed, in which the photoinduced charge transfer brings multiple advantages such as increased conductivity and electron mobility, as well as reduced work function. Average power conversion efficiency over 10% is achieved even when the thickness of the cathode interlayer and active layer is up to 100 and 300 nm, respectively.