Ligang Yuan

Kesterite is an attractive material for absorber layers in thin film photovoltaics. Solar cells based on kesterite have shown a substantial progress over the last decade; nevertheless, further improvements in device efficiency are pending due to the open-circuit voltage (V_oc) deficit (i.e., difference between the maximum V _oc that can be achieved according to Shockley–Queisser limit and actual V _oc from the device). In this study, the optoelectronic properties of the author's internal record Cu₂ZnSnSe₄ solar cell, which shows a power conversion efficiency of 11.4%, are presented. The device measurements reveal a V_oc deficit of 337 mV, which is one of the lowest V _oc deficits in the literature. Moreover, an unusual behavior for kesterite is observed: (i) photon energy of the photoluminescence emission and (ii) the extrapolated V _oc for 0 K are both matching the band gap region of the absorber. These results indicate a significant improvement in the recombination characteristics and absorber quality in comparison to other kesterite devices in literature.

Optoelectronic properties of an 11.4% Cu₂ZnSnSe₄ solar cell are presented. The device shows an unusual behavior for kesterite: (i) photon energy of the photoluminescence emission and (ii) the extrapolated V_oc for 0 K are both matching the band gap region of the absorber. These properties lead to one of the lowest V_oc deficits for kesterite in the literature.

In article 1705875, Zhao-Kui Wang, Peng-Fei Fang, Liang-Sheng Liao, and co-workers incorporate graphitic carbon nitride (g-C₃N₄) into the perovskite layer of perovskite solar cells. The additive retards the crystallization rate, improving crystalline quality and reducing the intrinsic defect density of the perovskite film. Increasing the fill factor from 0.65 to 0.74 yields a stable device with a power conversion efficiency of 19.49%.

In article 1703398, Jong-Hong Lu, Chih-Ping Chen and, co-workers demonstrate a Ag/ITO/Ag based microcavity (MC) structure for colorful organic photovoltaics applications. OPVs with an ultra-wide vivid color-gamut (blue, green, yellow-green, yellow, orange, and red), with PCEs as high as 8.2% for the yellow-green [CIE 1931: (0.364, 0.542)] device with a highest transmittance of 17.3% at 561 nm are demonstrated.

2D halide perovskites have recently been recognized as a promising avenue in perovskite solar cells (PSCs) in terms of encouraging stability and defect passivation effect. However, the efficiency (less than 15%) of ultrastable 2D Ruddlesden–Popper PSCs still lag far behind their traditional 3D perovskite counterparts. Here, a rationally designed 2D-3D perovskite stacking-layered architecture by in situ growing 2D PEA₂PbI₄ capping layers on top of 3D perovskite film, which drastically improves the stability of PSCs without compromising their high performance, is reported. Such a 2D perovskite capping layer induces larger Fermi-level splitting in the 2D-3D perovskite film under light illumination, resulting in an enhanced open-circuit voltage (V_oc) and thus a higher efficiency of 18.51% in the 2D-3D PSCs. Time-resolved photoluminescence decay measurements indicate the facilitated hole extraction in the 2D-3D stacking-layered perovskite films, which is ascribed to the optimized energy band alignment and reduced nonradiative recombination at the subgap states. Benefiting from the high moisture resistivity as well as suppressed ion migration of the 2D perovskite, the 2D-3D PSCs show significantly improved long-term stability, retaining nearly 90% of the initial power conversion efficiency after 1000 h exposure in the ambient conditions with a high relative humidity level of 60 ± 10%.

2D perovskite capping layers are grown in situ on top of the 3D perovskite film, leading to an enhanced efficiency of 18.5% in the stacking-layered 2D-3D perovskite solar cells (PSCs). Moreover, the unencapsulated 2D-3D PSCs show drastically improved long-term stability, retaining nearly 90% of the original efficiency after 1000 h exposure in a highly humid environment.

Hole transport layer (HTL) plays a critical role for achieving high performance solution-processed optoelectronics including organic electronics. For organic solar cells (OSCs), the inverted structure has been widely adopted to achieve prolonged stability. However, there are limited studies of p-type effective HTL on top of the organic active layer (hereafter named as top HTL) for inverted OSCs. Currently, p-type top HTLs are mainly 2D materials, which have an intrinsic vertical conduction limitation and are too thin to function as practical HTL for large area optoelectronic applications. In the present study, a novel self-assembled quasi-3D nanocomposite is demonstrated as a p-type top HTL. Remarkably, the novel HTL achieves ≈15 times enhanced conductivity and ≈16 times extended thickness compared to the 2D counterpart. By applying this novel HTL in inverted OSCs covering fullerene and non-fullerene systems, device performance is significantly improved. The champion power conversion efficiency reaches 12.13%, which is the highest reported performance of solution processed HTL based inverted OSCs. Furthermore, the stability of OSCs is dramatically enhanced compared with conventional devices. The work contributes to not only evolving the highly stable and large scale OSCs for practical applications but also diversifying the strategies to improve device performance.

A novel self-assembled quasi-3D nanocomposite is demonstrated to be an effective top hole transport layer (HTL) for both fullerene and non-fullerene inverted organic solar cells. Due to the better conductivity of this nanocomposite HTL, the thickness sensitivity issue of graphene oxide is addressed. Surface recombination is suppressed and the highest power conversion efficiency can reach 12.13%.

New light is shed on the previously known perovskite material, Cs₂Au₂I₆, as a potential active material for high-efficiency thin-film Pb-free photovoltaic cells. First-principles calculations demonstrate that Cs₂Au₂I₆ has an optimal band gap that is close to the Shockley–Queisser value. The band gap size is governed by intermediate band formation. Charge disproportionation on Au makes Cs₂Au₂I₆ a double-perovskite material, although it is stoichiometrically a single perovskite. In contrast to most previously discussed double perovskites, Cs₂Au₂I₆ has a direct-band-gap feature, and optical simulation predicts that a very thin layer of active material is sufficient to achieve a high photoconversion efficiency using a polycrystalline film layer. The already confirmed synthesizability of this material, coupled with the state-of-the-art multiscale simulations connecting from the material to the device, strongly suggests that Cs₂Au₂I₆ will serve as the active material in highly efficient, nontoxic, and thin-film perovskite solar cells in the very near future.

Cs₂Au₂I₆ is an interesting perovskite material having mixed-valence Au centers. The current state-of-the-art multiscale simulation shows that this old-but-new material is highly advantageous as a lead-free and stable material for very thin-film perovskite solar cells (PSCs). This work is expected to inspire experimentalists to realize this new concept in PSCs based on mixed-valence perovskites.

High-quality pinhole-free perovskite film with optimal crystalline morphology is critical for achieving high-efficiency and high-stability perovskite solar cells (PSCs). In this study, a p-type π-conjugated polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)-benzo[1,2-b:4,5-b′] dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl) benzo[1′,2′-c:4′,5′-c′] dithiophene-4,8-dione))] (PBDB-T) is introduced into chlorobenzene to form a facile and effective template-agent during the anti-solvent process of perovskite film formation. The π-conjugated polymer PBDB-T is found to trigger a heterogeneous nucleation over the perovskite precursor film and passivate the trap states of the mixed perovskite film through the formation of Lewis adducts between lead and oxygen atom in PBDB-T. The p-type semiconducting and hydrophobic PBDB-T polymer fills in the perovskite grain boundaries to improve charge transfer for better conductivity and prevent moisture invasion into the perovskite active layers. Consequently, the PSCs with PBDB-T modified anti-solvent processing leads to a high-efficiency close to 20%, and the devices show excellent stability, retaining about 90% of the initial power conversion efficiency after 150 d storage in dry air.

p-Type π-conjugated polymer is introduced during the anti-solvent process to form high-quality pinhole-free perovskite films. Traps are passivated through Lewis adducts between the lead and oxygen atoms in the polymer. The hydrophobic polymer protects the perovskite grain boundaries against moisture invasion. The perovskite solar cells show efficiency reaching 20%, and high stability under storage, thermal stress (85 °C), and white-light illumination.

Rapid progress in the power conversion efficiency (PCE) of polymer solar cells (PSEs) is beneficial from the factors that match the irradiated solar spectrum, maximize incident light absorption, and reduce photogenerated charge recombination. To optimize the device efficiency, a nanopatterned ZnO:Al₂O₃ composite film is presented as an efficient light- and charge-manipulation layer (LCML). The Al₂O₃ shells on the ZnO nanoparticles offer the passivation effect that allows optimal electron collection by suppressing charge-recombination loss. Both the increased refractive index and the patterned deterministic aperiodic nanostructure in the ZnO:Al₂O₃ LCML cause broadband light harvesting. Highly efficient single-junction PSCs for different binary blends are obtained with a peak external quantum efficiency of up to 90%, showing certified PCEs of 9.69% and 13.03% for a fullerene blend of PTB7:PC₇₁BM and a nonfullerene blend, FTAZ:IDIC, respectively. Because of the substantial increase in efficiency, this method unlocks the full potential of the ZnO:Al₂O₃ LCML toward future photovoltaic applications.

Highly efficient polymer solar cells based on nanopatterned ZnO:Al₂O₃ composite film achieve a peak external quantum efficiency up to 90% and a certified power conversion efficiency of 13.03%. Optical and electrical studies demonstrate enhanced light harvesting due to passivation- and dipole-induced suppression of charge recombination loss and broadband absorption enhancement.

Realizing over 10% efficiency in printed organic solar cells via scalable materials and less toxic solvents remains a grand challenge. In article number 1705485, Harald Ade and co-workers report chlorine-free, in-air blade-coating of a new photoactive combination, FTAZ:IT-M, which is able to yield an efficiency of nearly 11%, despite a high humidity of ≈50%.

Atomic configurations of superlattices of an organometal halide perovskite layer composed of a mixture of tetragonal and cubic phases are reported by Tae Woong Kim, Satoshi Uchida, Hiroshi Segawa, and co-workers in article number 1705230. The tetragonal and cubic phases are found to coexist at room temperature, and superlattices composed of a mixture of tetragonal and cubic phases are found to be self-organized without a compositional change. The fundamental crystallographic information of the organometal halide perovskite is shown and their new possibilities as promising materials for various applications are demonstrated.

Perovskite solar cells (PSCs) exhibit a series of distinctive features in their optoelectronic response which have a crucial influence on the performance, particularly for long-time response. Here, a survey of recent advances both in device simulation and optoelectronic and photovoltaic responses is provided, with the aim of comprehensively covering recent advances. Device simulations are included with clarifying discussions about the implications of classical drift–diffusion modeling and the inclusion of ionic charged layers near the outer carrier selective contacts. The outcomes of several transient techniques are summarized, along with the discussion of impedance and capacitive responses upon variation of bias voltage and irradiance level. In relation to the capacitive response, a discussion on the J–V curve hysteresis is also included. Although alternative models and explanations are included in the discussion, the review relies upon a key mechanism able to yield most of the rich experimental responses. Particularly for state-of-the-art solar cells exhibiting efficiencies around or exceeding 20%, outer interfaces play a determining role on the PSC's performance. The ionic and electronic kinetics in the vicinity of the interfaces, coupled to surface recombination and carrier extraction mechanisms, should be carefully explored to progress further in performance enhancement.

A survey of recent advances both in device simulation and optoelectronic and photovoltaic responses is provided, with the aim of comprehensively covering recent progress. The outcomes of several transient techniques are summarized, along with the discussion of impedance and capacitive responses upon variation of bias voltage and irradiance level. The J–V curve hysteresis effect is also discussed.

In this report, highly efficient and humidity-resistant perovskite solar cells (PSCs) using two new small molecule hole transporting materials (HTM) made from a cost-effective precursor anthanthrone (ANT) dye, namely, 4,10-bis(1,2-dihydroacenaphthylen-5-yl)-6,12-bis(octyloxy)-6,12-dihydronaphtho[7,8,1,2,3-nopqr]tetraphene (ACE-ANT-ACE) and 4,4′-(6,12-bis(octyloxy)-6,12-dihydronaphtho[7,8,1,2,3-nopqr]tetraphene-4,10-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (TPA-ANT-TPA) are presented. The newly developed HTMs are systematically compared with the conventional 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamino)-9,9′-spirbiuorene (Spiro-OMeTAD). ACE-ANT-ACE and TPA-ANT-TPA are used as a dopant-free HTM in mesoscopic TiO₂/CH₃NH₃PbI₃/HTM solid-state PSCs, and the performance as well as stability are compared with Spiro-OMeTAD-based PSCs. After extensive optimization of the metal oxide scaffold and device processing conditions, dopant-free novel TPA-ANT-TPA HTM-based PSC devices achieve a maximum power conversion efficiency (PCE) of 17.5% with negligible hysteresis. An impressive current of 21 mA cm⁻² is also confirmed from photocurrent density with a higher fill factor of 0.79. The obtained PCE of 17.5% utilizing TPA-ANT-TPA is higher performance than the devices prepared using doped Spiro-OMeTAD (16.8%) as hole transport layer at 1 sun condition. It is found that doping of LiTFSI salt increases hygroscopic characteristics in Spiro-OMeTAD; this leads to the fast degradation of solar cells. While, solar cells prepared using undoped TPA-ANT-TPA show dewetting and improved stability. Additionally, the new HTMs form a fully homogeneous and completely covering thin film on the surface of the active light absorbing perovskite layers that acts as a protective coating for underlying perovskite films. This breakthrough paves the way for development of new inexpensive, more stable, and highly efficient ANT core based lower cost HTMs for cost-effective, conventional, and printable PSCs.

First time low-cost anthanthrone dye based hole transporting materials (HTMs) 4,10-bis(1,2-dihydroacenaphthylen-5-yl)-6,12-bis(octyloxy)-6,12-dihydronaphtho[7,8,1,2,3-nopqr]tetraphene (ACE-ANT-ACE) and 4,4′-(6,12-bis(octyloxy)-6,12-dihydronaphtho[7,8,1,2,3-nopqr]tetraphene-4,10-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (TPA-ANT-TPA) end capped with dihydroacenaphthylene and triphenyleamine groups are designed and synthesized, respectively. Among both, dopant-free TPA-ANT-TPA cut-rate HTM ($67 g⁻¹) exhibits higher performance with 17.5% efficiency and retains respectable performance after 50 h in 58% relative humidity than conventional expensive 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamino)-9,9′-spirbiuorene.

Highly stable and efficient two-dimensional (2D) perovskite solar cells are fabricated by modifying the solution deposition process. The modification of deposition process enhances the efficiency of C₆H₅(CH₂)₂NH₃)₂(CH₃NH₃)₃ Pb₄I₁₃ perovskite solar cell by ≈28% with respect to the conventional deposition. This study presents a comprehensive investigation of moisture and thermal stability of 2D perovskites, synthesized using large aromatic alkylammonium as spacer cation.

The recent progress in low-dimensional perovskite solar cells are reviewed, focusing on the enhanced stability and reduced toxicity. The versatile low-dimensional perovskites also hold great promise to be alternative candidates for various optoelectronic applications.

Perovskite solar cells have attracted much attention in the last few years due to their high power conversion efficiency (PCE) and solution based printing process compatibility. The preparation of a metal electrode with printing process is, however, a big challenge for perovskite solar cells, which is, on the one hand, due to the sensitivity of the perovskite film, and on the other hand, due to the poor interconnection between electrode buffer layer and the printed metal electrode. This problem was solved in article number 1700184 by Qun Luo, Jian Lin, Liyi Shi,Chang-Qi Ma and co-workers, by inserting a thin polyethylenimine (PEI) layer. Therefore, achieving high performance semi-transparent perovskite solar cells with ink-jet printed silver nanowire top electrode.

In this work, a large-area (15.6 × 15.6 cm²) PEDOT:PSS/c-Si heterojunction solar cells with phosphorus-diffused emitter and screen-printed metal contacts on the front side is presented. The best-performing solar cell achieves an open-circuit voltage V_oc of 656 mV and a short-circuit current density J_sc of 38.7 mA cm⁻², and an efficiency of 20.2%. Furthermore, a low-temperature metal paste for the PEDOT:PSS metallization is developed.