ZiQi Sun

Charge transport in organic photovoltaic (OPV) devices is often characterized by steady-state mobilities. However, the suitability of steady-state mobilities to describe charge transport has recently been called into question, and it has been argued that dispersion plays a significant role. In this paper, the importance of the dispersion of charge carrier motion on the performance of organic photovoltaic devices is investigated. An experiment to measure the charge extraction time under realistic operating conditions is set up. This experiment is applied to different blends and shows that extraction time is directly related to the geometrical average of the steady-state mobilities. This demonstrates that under realistic operating conditions the steady-state mobilities govern the charge extraction of OPV and gives a valuable insight in device performance.

Charge transport in organic photovoltaic devices is often characterized by steady-state mobilities. However, the suitability of steady-state mobilities to describe charge transport has recently been called into question and it has been argued that dispersion plays a significant role. In this paper, the importance of the dispersion of charge carrier motion on the performance of organic photovoltaic devices is investigated.

Triple-junction device architectures represent a promising strategy to highly efficient organic solar cells. Accurate characterization of such devices is challenging, especially with respect to determining the external quantum efficiency (EQE) of the individual subcells. The specific light bias conditions that are commonly used to determine the EQE of a subcell of interest cause an excess of charge generation in the two other subcells. This results in the build-up of an electric field over the subcell of interest, which enhances current generation and leads to an overestimation of the EQE. A new protocol, involving optical modeling, is developed to correctly measure the EQE of triple-junction organic solar cells. Apart from correcting for the build-up electric field, the effect of light intensity is considered with the help of representative single-junction cells. The short-circuit current density (J_SC) determined from integration of the EQE with the AM1.5G solar spectrum differs by up to 10% between corrected and uncorrected protocols. The results are validated by comparing the EQE experimentally measured to the EQE calculated via optical-electronic modeling, obtaining an excellent agreement.

The external quantum efficiency of triple-junction cells is accurately measured following a new protocol that takes into account light bias and voltage bias. Integration of the external quantum efficiency to determine the short-circuit current density matches with the value under simulated AM1.5G illumination conditions and results in a power conversion efficiency of 9.77 ± 0.29%.

Highly crystalline conjugated polymers represent a key material for producing high-performance thick-active-layer polymer solar cells (PSCs). However, despite their potential, a limited number of crystalline polymers are used in PSCs because of the lack of highly coplanar acceptor building blocks and insufficient light absorptivity (α < 10⁵) of most donor (D)–acceptor (A)-type polymers. This study reports a series of novel 3,7-di(thiophen-2-yl)-1,5-naphthyridine-2,6-dione (NTDT) acceptor-based conjugated polymers, PNTDT-2T, PNTDT-TT, and PNTDT-2F2T, synthesized with 2,2′-bithiophene (2T), thieno[3,2-b]thiophene (TT), and 3,3′-difluoro-2,2′-bithiophene (2F2T) donor units, respectively. PNTDT-2F2T exhibits superior polymer crystallinity and a much higher absorption coefficient than those of PNTDT-2T or PNTDT-TT because of adequate matching between highly coplanar A (NTDT) and D (2F2T) building blocks. A bulk heterojunction solar cell based on PNTDT-2F2T exhibits a power conversion efficiency of up to 9.63%, with a high short circuit current of 18.80 mA cm⁻² and fill factor of 0.70, when a thick active layer (>200 nm) is used, without postfabrication hot processing. The findings demonstrate that the polymer crystallinity and absorption coefficient can be effectively controlled by selecting appropriate D and A building blocks, and that NTDT is a novel and versatile A building block for highly efficient thick-active-layer PSCs.

A novel 1,5-naphthyridine-2,6-dione-based conjugated polymer is developed and applied as a donor in the active layers of efficient polymer solar cells. PNTDT-2F2T exhibits outstanding polymer crystallinity and absorption coefficient. A polymer solar cell device made using PNTDT-2F2T exhibits a high power conversion efficiency (9.63%) with a thick active layer (>200 nm).

To advance polymer solar cells (PSCs) toward real-world applications, it is crucial to develop materials that are compatible with a low-cost large-scale manufacturing technology. In this context, a practically useful polymer should fulfill several critical requirements: the capability to provide high power conversion efficiencies (PCEs) via low-cost fabrication using environmentally friendly solvents under mild thermal conditions, resulting in an active layer that is thick enough to minimize defects in large-area films. Here, the development of new photovoltaic polymers is reported through rational molecular design to meet these requirements. Benzodithiophene (BDT)-difluorobenzoxadiazole (ffBX)-2-decyltetradecyl (DT), a wide-bandgap polymer based on ffBX and BDT emerges as the first example that fulfills the qualifications. When blended with a low-cost acceptor (C₆₀-fullerene derivative), BDT-ffBX-DT produces a PCE of 9.4% at active layer thickness over 250 nm. BDT-ffBX-DT devices can be fabricated from nonhalogenated solvents at low processing temperature. The success of BDT-ffBX-DT originates from its appropriate electronic structure and charge transport characteristics, in combination with a favorable face-on orientation of the polymer backbone in blends, and the ability to form proper phase separation morphology with a fibrillar bicontinuous interpenetrating network in bulk-heterojunction films. With these characteristics, BDT-ffBX-DT represents a meaningful step toward future everyday applications of polymer solar cells.

Two new conjugated polymers based on difluorobenzoxadiazole bring real-world applications of polymer solar cells closer. They integrate multiple advantages including high power conversion efficiency built on low-cost acceptors, allowing thick active layers, and processability from green solvents under mild conditions. Particularly, benzodithiophene-difluorobenzoxadiazole-2-decyltetradecyl (BDT-ffBX-DT) is a champion in meeting a comprehensive list of prerequisites for future application of polymer solar cells.

In very recent years, growing efforts have been devoted to the development of all-polymer solar cells (all-PSCs). One of the advantages of all-PSCs over the fullerene-based PSCs is the versatile design of both donor and acceptor polymers which allows the optimization of energy levels to maximize the open-circuit voltage (V_oc). However, there is no successful example of all-PSCs with both high V_oc over 1 V and high power conversion efficiency (PCE) up to 8% reported so far. In this work, a combination of a donor polymer poly[4,8-bis(5-(2-octylthio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-(5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione)-1,3-diyl] (PBDTS-TPD) with a low-lying highest occupied molecular orbital level and an acceptor polymer poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-thiophene-2,5-diyl] (PNDI-T) with a high-lying lowest unoccupied molecular orbital level is used, realizing high-performance all-PSCs with simultaneously high V_oc of 1.1 V and high PCE of 8.0%, and surpassing the performance of the corresponding PC₇₁BM-based PSCs. The PBDTS-TPD:PNDI-T all-PSCs achieve a maximum internal quantum efficiency of 95% at 450 nm, which reveals that almost all the absorbed photons can be converted into free charges and collected by electrodes. This work demonstrates the advantages of all-PSCs by incorporating proper donor and acceptor polymers to boost both V_oc and PCEs.

High-performance all-polymer solar cells with high V_oc of 1.1 V and PCE of 8.0% are realized by incorporating a pair of the donor polymer poly[4,8-bis(5-(2-octylthio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-(5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione)-1,3-diyl] and acceptor polymer poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-thiophene-2,5-diyl]. The simultaneously high V_oc and power conversion efficiency stem from the low photon energy loss and high internal quantum efficiency near unity.

A novel atomic stacking transporting layer (ASTL) based on 2D atomic sheets of titania (Ti_1−δO₂) is demonstrated in organic–inorganic lead halide perovskite solar cells. The atomically thin ASTL of 2D titania, which is fabricated using a solution-processed self-assembly atomic layer-by-layer deposition technique, exhibits the unique features of high UV transparency and negligible (or very low) oxygen vacancies, making it a promising electron transporting material in the development of stable and high-performance perovskite solar cells. In particular, the solution-processable atomically thin ASTL of 2D titania atomic sheets shows superior inhibition of UV degradation of perovskite solar cell devices, compared to the conventional high-temperature sintered TiO₂ counterpart, which usually causes the notorious instability of devices under UV irradiation. The discovery opens up a new dimension to utilize the 2D layered materials with a great variety of homostructrual or heterostructural atomic stacking architectures to be integrated with the fabrication of large-area photovoltaic or optoelectronic devices based on the solution processes.

The self-assembly atomic stacking transporting layer based on 2D titania atomic sheets exhibits unique advantages when used as an electron transporting layer for the development of stable and high-performance perovskite solar cells. The advantages are shown to include high UV transparency, negligible (or very low) oxygen vacancies, and solution processability.

In article number 1701613, Milena P. Arciniegas, Liberato Manna, and co-workers combine state-of-the-art laser writing with a novel procedure allowing the in situ formation of methylammonium ions (the building blocks of perovskites) to localize stable MAPbBr₃ crystals. By the laser-induced heating of the liquid precursors, this approach produces luminescent and electrically conductive perovskite crystals following pre-designed patterns.

It is a great challenge to obtain broadband response perovskite photodetectors (PPDs) due to the relatively large bandgaps of the most used methylammonium lead halide perovskites. The response range of the reported PPDs is limited in the ultraviolet–visible range. Here, highly sensitive PPDs are successfully fabricated with low bandgap (≈1.25 eV) (FASnI₃)_0.6(MAPbI₃)_0.4 perovskite as active layers, exhibiting a broadband response from 300 to 1000 nm. The performance of the PPDs can be optimized by adjusting the thicknesses of the perovskite and C₆₀ layers. The optimized PPDs with 1000 nm thick perovskite layer and 70 nm thick C₆₀ layer exhibit an almost flat external quantum efficiency (EQE) spectrum from 350 to 900 nm with EQE larger than 65% under −0.2 V bias. Meanwhile, the optimized PPDs also exhibit suppressed dark current of 3.9 nA, high responsivity (R) of over 0.4 A W⁻¹, high specific detectivity (D*) of over 10¹² Jones in the near-infrared region under −0.2 V. Such highly sensitive broadband response PPDs, which can work well as self-powered conditions, offer great potential applications in multicolor light detection.

Highly sensitive perovskite photodetectors (PPDs) with broadband response from ultraviolet to the near-infrared region are achieved with low-bandgap (≈1.25 eV) (FASnI₃)_0.6(MAPbI₃)_0.4 perovskite as active layer. The optimized PPDs with 1000 nm thick perovskite and 70 nm thick C₆₀ electron transport layer exhibit an almost flat response from 350 to 900 nm with external quantum efficiency larger than 65% under −0.2 V bias.

Perovskite solar cells (PSCs) based on cesium (Cs)- and rubidium (Rb)-containing perovskite films show highly reproducible performance; however, a fundamental understanding of these systems is still emerging. Herein, this study has systematically investigated the role of Cs and Rb cations in complete devices by examining the transport and recombination processes using current–voltage characteristics and impedance spectroscopy in the dark. As the credibility of these measurements depends on the performance of devices, this study has chosen two different PSCs, (MAFACs)Pb(IBr)₃ (MA = CH₃NH₃⁺, FA = CH(NH₂)₂⁺) and (MAFACsRb)Pb(IBr)₃, yielding impressive performances of 19.5% and 21.1%, respectively. From detailed studies, this study surmises that the confluence of the low trap-assisted charge-carrier recombination, low resistance offered to holes at the perovskite/2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene interface with a low series resistance (R_s), and low capacitance leads to the realization of higher performance when an extra Rb cation is incorporated into the absorber films. This study provides a thorough understanding of the impact of inorganic cations on the properties and performance of highly efficient devices, and also highlights new strategies to fabricate efficient multiple-cation-based PSCs.

The confluence of low trap-assisted charge-carrier recombination, low resistance offered to holes at the perovskite/2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene interface with a low series resistance (R_S) and a lower value of charge storage, leads to the realization of higher photovoltaic performance when an extra cation (Rb) is incorporated into the perovskite films.

This study reports an effective amidine-type n-dopant of 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) that can universally dope electron acceptors, including PC₆₁BM, N2200, and ITIC, by mixing the dopant with the acceptors in organic solvents or exposing the acceptor films in the dopant vapor. The doping mechanism is due to its strong electron-donating property that is also confirmed via the chemical reduction of PEDOT:PSS (yielding color change). The DBU doping considerably increases the electrical conductivity and shifts the Fermi levels up of the PC₆₁BM films. When the DBU-doped PC₆₁BM is used as an electron-transporting layer in perovskite solar cells, the n-doping removes the “S-shape” of J–V characteristics, which leads to the fill factor enhancement from 0.54 to 0.76. Furthermore, the DBU doping can effectively lower the threshold voltage and enhance the electron mobility of PC₆₁BM-based n-channel field-effect transistors. These results show that the DBU can be a promising n-dopant for solution-processed electronics.

An effective, solution-processed amidine-type n-dopant of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which can universally dope electron acceptor materials, including PC₆₁BM, N2200, and ITIC, is reported. The DBU doping can enhance the performance of the perovskite solar cells and the electron mobility of the field-effect transistors.

Dramatic advances in perovskite solar cells (PSCs) and the blossoming of wearable electronics have triggered tremendous demands for flexible solar-power sources. However, the fracturing of functional crystalline films and transmittance wastage from flexible substrates are critical challenges to approaching the high-performance PSCs with flexural endurance. In this work, a nanocellular scaffold is introduced to architect a mechanics buffer layer and optics resonant cavity. The nanocellular scaffold releases mechanical stresses during flexural experiences and significantly improves the crystalline quality of the perovskite films. The nanocellular optics resonant cavity optimizes light harvesting and charge transportation of devices. More importantly, these flexible PSCs, which demonstrate excellent performance and mechanical stability, are practically fabricated in modules as a wearable solar-power source. A power conversion efficiency of 12.32% for a flexible large-scale device (polyethylene terephthalate substrate, indium tin oxide-free, 1.01 cm²) is achieved. This ingenious flexible structure will enable a new approach for development of wearable electronics.

A nanocellular scaffold is introduced to construct a mechanics buffer layer and optics resonant cavity in a flexible perovskite solar cell. A power conversion efficiency of 12.32% is achieved with a flexible, large-scale device (polyethylene terephthalate substrate, indium tin oxide-free, 1.01 cm²). Moreover, the devices, which demonstrate excellent performance and mechanical stability, are practically fabricated in modules for a wearable solar-power source.

The mechanism behind the temperature dependence of the device performance in hybrid perovskite solar cells (HPSCs) is investigated systematically. The power conversion efficiency (PCE) of the reference cell using [60]PCBM as electron extraction layer (EEL) drops significantly from 11.9% at 295 K to 7% at 180 K. The deteriorated charge carrier extraction is found as the dominant factor causing this degradation. Temperature dependent spectroscopy and charge transport studies demonstrate that the poor electron transport in the [60]PCBM EEL at low temperature leads to inefficient charge carrier extraction. It is further demonstrated that the n-type doping of [60]PCBM EEL or the use of an EEL (fulleropyrrolidine with a triethylene glycol monoethyl ether side chain) with higher electron transport capability is an effective strategy to achieve HPSCs working efficiently over a broad temperature range. The devices fabricated with these highly performing EELs have PCEs at 180 K of 16.7% and 18.2%, respectively. These results support the idea that the temperature dependence of the electron transport in the EELs limits the device performance in HPSCs, especially at lower temperatures and they also give directions toward further improvement of the PCE of HPSCs at realistic operating temperatures.

The temperature dependence of the figures of merit of hybrid perovskite solar cells (HPSCs) is dominated by the electron extraction layer (EEL). At low temperature, the device using [60]PCBM as EEL shows significant lowering of the performance due to the poor electron transport capability of [60]PCBM. By n-type doping [60]PCBM, highly efficient HPSCs over a broad temperature range are achieved.

In article number 1602812, Jung-Yong Lee, Bumjoon J. Kim, and co-workers investigate nonconjugated polymer additives (nPAs) for highly efficient and stable polymer solar cells (PSCs). The P2VP nPA self-assembles vertically on the ZnO surface via a single coating process for the deposition of active materials. The self-assembled P2VP reduces the work function and surface defect density of ZnO, which leads to efficient and stable PSCs with up to 11.14% efficiency.