ZiQi Sun

A perovskite solar cell (PSC) employing an organic–inorganic lead halide perovskite light harvester, seeded in 2009 with power conversion efficiency (PCE) of 3.8% and grown in 2011 with PCE of 6.5% in dye-sensitized solar cell structure, has received great attention since the breakthrough reports ≈10% efficient solid-state PCSs demonstrating 500 h stability. Developments of device layout and high-quality perovskite film eventually lead to a PCE over 22%. As of October 31, 2017, the highest PCE of 22.7% is listed in an efficiency chart provided by NREL. In this Review, the methodologies to obtain highly efficient PSCs are described in detail. In order to achieve a PCE of over 20% reproducibly, key technologies are disclosed from the viewpoint of precursor solution chemistry, processing for defect-free perovskite films, and passivation of grain boundaries. Understanding chemical species in precursor solution, crystal growth kinetics, light–matter interaction, and controlling defects is expected to give important insights into not only reproducible production of high PCE over 20% but also further enhancement of the PCE of PCSs.

A better understanding of precursor chemistry and the crystal growth mechanism can lead to power conversion efficiency of perovskite solar cells as high as 22.7%.

In article number 1701935 by Taiho Park and co-workers, tetraethylene glycol (TEG) group is incorporated into a well-known high mobility polymer backbone to improve its solubility. The polymer, PTEG, exhibits high mobility and solubility in common organic solvents, showing the highest efficiency (19.8%) in the planar perovskite solar cell.

In the field of organic solar cells (OSCs), tandem structure devices exhibit very attractive advantages for improving power conversion efficiency (PCE). In addition to the well researched novel pair of active layers in different subcells, the construction of interconnecting layer (ICL) also plays a critical role in achieving high performance tandem devices. In this work, a new way of achieving environmentally friendly solvent processed polymeric ICL by adopting poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-5,5′-bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N′-di(2-ethylhexyl)imide] (PNDIT-F3N) blended with poly(ethyleneimine) (PEI) as the electron transport layer (ETL) and PEDOT:PSS as the hole transport layer is reported. It is found that the modification ability of PNDIT-F3N on PEDOT can be linearly tuned by the incorporation of PEI, which offers the opportunity to study the charge recombination behavior in ICL. At last, tandem OSC with highest PCE of 12.6% is achieved, which is one of the best tandem OSCs reported till now. These results offer a new selection for constructing efficient ICL in high performance tandem OSCs and guide the way of design new ETL materials for ICL construction, and may even be integrated in future printed flexible large area module device fabrication with the advantages of environmentally friendly solvent processing and thickness insensitivity.

A new polymeric interconnecting layer (ICL) based on poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-5,5′-bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N′-di(2-ethylhexyl)imide]: poly(ethyleneimine)/PEDOT:PSS is developed and applied for the fabrication of high performance tandem organic solar cells (OSCs). Tandem OSCs employing this ICL achieve a high power conversion efficiency of 12.6% with ICL thickness of 60 nm and even reach to 11.3% with ICL thickness of 140 nm.

A novel wide-bandgap electron-donating copolymer containing an electron-deficient, difluorobenzotriazole building block with a siloxane-terminated side chain is developed. The resulting polymer, poly{(4,8-bis(4,5-dihexylthiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-4,7-di(thiophen-2-yl)-5,6-difluoro-2-(6-(1,1,1,3,5,5,5-heptamethyltri-siloxan-3-yl)hexyl)-2H-benzo[d][1,2,3]triazole} (PBTA-Si), is used to successfully fabricate high-performance, ternary, all-polymer solar cells (all-PSCs) insensitive to the active layer thickness. An impressively high fill factor of ≈76% is achieved with various ternary-blending ratios. The optimized all-PSCs attain a power conversion efficiency (PCE) of 9.17% with an active layer thickness of 350 nm and maintain a PCE over 8% for thicknesses over 400 nm, which is the highest reported efficiency for thick all-PSCs. These results can be attributed to efficient charge transfer, additional energy transfer, high and balanced charge transport, and weak recombination behavior in the photoactive layer. Moreover, the photoactive layers of the ternary all-PSCs are processed in a nonhalogenated solvent, 2-methyltetrahydrofuran, which greatly improves their compatibility with large-scale manufacturing.

A novel electron-donating copolymer, PBTA-Si, containing a benzotriazole building block with a siloxane-functionalized side chain, is developed and used to fabricate thick-film all-polymer solar cells (all-PSC). By means of ternary blending, the all-PSCs attain a power conversion efficiency of 9.17% with a 350 nm thick active layer and 8.34% with a thickness of 420 nm.

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 article number 1701683, In Hwan Jung, Sung-Yeon Jang, and co-workers report the synthesis of ‘high-efficiency low-temperature processed perovskite solar cells’ by the combination of two techniques; the modification of solution-processed ZnO using a self-assembled monolayer, and the fabrication of perovskite layers by a sequential deposition method. This technique may pave the way to efficiently reducing the processing temperature of perovskite solar cells.

In article number 1702235, Taiho Park and co-workers demonstrate that the efficiency of a planar-type perovskite solar cell is increased by the addition of CuI ionic salt to the TiO₂ electron transport layer. Due to the characteristics of CuI (Visualized on the cover as yellow and red spheres), the electrons from perovskite layer (pink octahedra) show fast extraction to TiO₂ layer (blue and purple spheres).

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.

Heterojunctions of carbon nanotubes interfaced with silicon respond to light illumination and can be operated in the power regime as solar cells. Very significant advances have been made in the last 5 years both in terms of headline performance values and in fundamental understanding of the underlying operating principles, as well as the sophistication of the devices and studies being reported. The body of literature is growing rapidly, and the latest power conversion efficiency and active area records have now reached over 17% and 2 cm², respectively. Thus, the authors believe that it is now a useful time for an evaluation of the current state-of-the-art and challenges going forward, as well as for a comprehensively updated review of progress made in the field. In addition, the authors provide a summary of the various fabrication schemes that have been used, analysis of some of the major device structure–property relationships revealed by comparison of published works, and a thorough breakdown of the various factors involved in improving performance, as well as a critical assessment of the real opportunities that may exist for this technology in the context of the wider silicon photovoltaics industry.

The significant progress that has been made in improving the performance and stability of carbon nanotube–silicon heterojunction solar cells, and in understanding of the operating principles, is discussed. In addition, a comprehensive review of the literature, a roadmap for future performance improvement, and a critical assessment of the opportunities for this rapidly developing field in the wider context of the silicon photovoltaics industry are provided.

This study proposes a novel strategy of controllable deamination of Co–NH₃ complexes in a system containing Ni(OH)₂ to synthesize ultrasmall ternary oxide nanoparticles (NPs), NiCo₂O₄. Through this approach, ultrasmall (5 nm on average) and well-dispersed NiCo₂O₄ NPs without exotic ligands are obtained, which enables the formation of uniform and pin-hole free films. The tightly covered NiCo₂O₄ films also facilitate the formation of large perovskite grains and thus reduce film defects. The results show that with the NiCo₂O₄ NPs as the hole transport layer (HTL), the perovskite solar cells reach a high power conversion efficiency (PCE) of 18.23% and a promising stability (maintained ≈90% PCE after 500 h light soaking). To the best of the author's knowledge, it is the first time that spinel NiCo₂O₄ NPs have been applied as hole transport layer in perovskite solar cells successfully. This work not only demonstrates the potential applications of ternary oxide NiCo₂O₄ as HTLs in hybrid perovskite solar cells but also provides an insight into the design and synthesis of ultrasmall and ligand-free NPs HTLs to enable cost-effective photovoltaic devices.

An ultrasmall and well-dispersed ternary oxide of NiCo₂O₄ nanoparticles, achieved by a new strategy of controllable deamination of the Co–NH₃ complexes in a system containing Ni(OH)₂ , facilitate the formation of large perovskite grains, which is first applied as hole transport layer for highly efficient perovskite solar cells.

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.

The recent rapid increase in efficiency of organic–inorganic perovskite solar cells (PSCs) has resulted in a need to develop a clear understanding of their stability and working mechanisms. In particular, it has been suggested that ion migration contributes to the commonly observed hysteresis in the current–voltage measurements of PSCs, but the rate of ion migration and its effects on the electronic properties of PSCs remain to be addressed. In this work, electron-beam-induced current (EBIC) is used to directly map changes in local current extraction in organic–inorganic PSCs under applied voltage. By combining EBIC mapping, standard current–voltage measurements, and external quantum efficiency measurements, it is shown that between the two potential roles that point defects play in device enhancement under voltage biasing, the effects caused by defect-mediated ion migration outweigh the effects from the filling of trap states caused by these defects. Evidence is also provided for ion migration preferentially at local features such as extended defects. The measured timescale of tens of seconds for migration across a full device imply that ion migration contributes indirectly to the electronic capacitance of perovskite devices through interface charging.

Electron-beam induced current measurements are used to map local current extraction in organic–inorganic perovskite solar cells. Current extraction before and after prebiasing of organic–inorganic perovskite solar cell devices and quantification of corresponding current decay rates support a model of mixed ionic and electronic processes contributing to the dependence of perovskite solar cells on previous biasing conditions.

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.

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.

Wide-bandgap (WBG) formamidinium–cesium (FA-Cs) lead iodide–bromide mixed perovskites are promising materials for front cells well-matched with crystalline silicon to form tandem solar cells. They offer avenues to augment the performance of widely deployed commercial solar cells. However, phase instability, high open-circuit voltage (V_oc) deficit, and large hysteresis limit this otherwise promising technology. Here, by controlling the crystallization of FA-Cs WBG perovskite with the aid of a formamide cosolvent, light-induced phase segregation and hysteresis in perovskite solar cells are suppressed. The highly polar solvent additive formamide induces direct formation of the black perovskite phase, bypassing the yellow phases, thereby reducing the density of defects in films. As a result, the optimized WBG perovskite solar cells (PSCs) (E_g ≈ 1.75 eV) exhibit a high V_oc of 1.23 V, reduced hysteresis, and a power conversion efficiency (PCE) of 17.8%. A PCE of 15.2% on 1.1 cm² solar cells, the highest among the reported efficiencies for large-area PSCs having this bandgap is also demonstrated. These perovskites show excellent phase stability and thermal stability, as well as long-term air stability. They maintain ≈95% of their initial PCE after 1300 h of storage in dry air without encapsulation.

The highly polar solvent additive, formamide, enables phase-selective crystallization in wide-bandgap (WBG) perovskites. By suppressing the formation of non-perovskite phases, the WBG perovskites (E_g ≈ 1.75 eV) exhibited excellent light-induced phase-, thermal, and air stability, as well as device performance with a high V_oc of 1.23 V and reduced hysteresis, with a power conversion efficiency (PCE) of 17.8%.

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.