Enormous efforts have been put into different aspects of perovskite solar cells (PSCs) and the progress has been incredibly fast on all fronts. The background, on-going R&Ds, and future direction of PSC research have been reported by our group as a comprehensive review.¹ Despite the current record efficiency of 25.5%, PSCs face serious challenges of practical stability and durability required for industrialization. Compositional engineering of lead halide perovskites using modulator molecules and mixing 2D and 3D structures has improved the stability of perovskites against heat and moisture. However, organic cations in halide perovskites (methylammonium, etc.) and use of diffusible ionic dopants in hole transport materials (HTMs) are responsible for low stability of perovskites at high temperatures (>120^oC). In this respect, use of all-inorganic perovskite materials and dopant-free HTMs is highly desired.² We have conducted the device fabrication in this direction using CsPbI₃ and CsPbI₂Br as solution-processed perovskite films. The perovskite absorbers were made into junction with dopant-free HTMs to develop thermally stable PSCs. By chemical engineering to improve the quality of interfacial structure at the hetero-junction, all-inorganic and dopant-free PSCs yielded power conversion efficiency up to 15%. Further we could enhance the open-circuit voltage (Voc) of this PSC up to 1.42V, which is the highest Voc ever achieved with a single perevskite cell.³ In preparation of all-inorganic perovskites, a big challenge should also be directed to development of lead-free materials for environmental safety in practical applications.²

   Among extensive applications of PSCs for outdoor and indoor power generation, use of PSCs in space environments is promising because thin perovskite photovoltaic films demonstrate high stability and tolerance against exposure to high energy particle irradiations (proton and electron beams).⁴ Thin absorbers (<500 nm) avoid accumulation of particles and due to intrinsic defect tolerant nature of perovskites, radiation-induced collision damage is highly suppressed. Our current progress in making perovskite photovoltaics and future perspectives will be discussed.

1. A. K. Jena, A. Kulkarni, and T. Miyasaka, Chem. Rev. 2019, 119, 3036–3103.
2. T. Miyasaka, A. Kulkarni, G. M. Kim, S. Oez and A. K. Jena, Adv. Energy Mat., 2019, 1902500.
3. G. Zhanlin, T. Miyasaka, et al. J. Am. Chem. Soc. 2020, 142, 21, 9725–9734.
4. Y. Miyazawa, M. Ikegami, H.-W. Chen, T. Ohshima, M. Imaizumi, K. Hirose, and T. Miyasaka, iScience 2018, 2, 148-155.

Lead halide perovskite is a new type of semiconductor optoelectronic material, which owns large absorption coefficient, long diffusion length, which make it as an excellent photovoltaic material. Perovskite solar cells with different bandgaps of absorber need to be studied for making efficient single or tandem solar cells. In this talk, we will show: 1) By contact engineering, for the bandgap around 1.5 eV, we have achieved close to 25% efficiency, 2) By the composition engineering, we have pushed the PCE of inorganic perovskite CsPbI₃ based solar cells to 20% efficiency, 3) According to the defect engineering, over 20% efficiency of the perovskite solar cells have been demonstrated for the perovskite bandgap around 1.35 eV.

Over more than two decades of research, organic solar cells have achieved tremendous progresses in materials & device engineering and applications. For further advance, the power conversion efficiencies (PCEs) of organic solar cells need to be substantially improved. 

Inspired by the recent success in non-fullerene electron acceptors (NFAs), we have developed a design strategy defined as “A-DA¢D-A” to obtain a series of high-performing NFAs, called as Y series. D = electron donor unit while A and A¢ = electron acceptor unit. The key to this molecular innovation is introducing an electron-deficient moiety (A¢) such as benzotriazole or benzothiadiazole into the central fused ring. Generally, these electron acceptors show extended absorption in the NIR region and provide considerably low energy losses in organic solar cells, hence having set new records for the certified power conversion efficiencies by National Renewable Energy Laboratory (NREL).

It is worth mentioned that our research on these newly designed electron acceptors has attracted extensive attention. For instance, the research paper on the Y6 acceptor (Joule, 2019, 3, 1140) was cited over 1000 times by the others within a very short time since its publication. More importantly, the certified power conversion efficiency of more than 17% has been reported by our fellow researchers based on the commercially available Y6. The underlying role of these acceptors has been actively investigated at home and abroad. While first achieving the 15% PCE in the single-junction solar cells, Y6 appears to be a universal electron acceptor and contributes to developing semi-transparent and flexible organic solar cells.

Keywords: Organic solar cells; Power conversion efficiency; Electron acceptor; Bulk heterojunction

Metal-halide perovskites are one of the most promising active materials for photovoltaic and light-emitting technologies, due to their excellent optoelectronic properties and thin films fabrication versatility. In less than 10 years, perovskite solar cells (PSCs) have achieved record power conversion efficiency (PCE) of 25.2% [1] presently demonstrated over an active area much smaller than 1 cm² and a continued focus on improving the operational stability [2-5]. The rapid progress has triggered the interest of transferring the existing technology from small area PSCs into large-area perovskite solar modules (PSMs), necessary for industrial expansion.

Recently we have demonstrated highly efficient, large area, planar PSCs with uniform MAPbI₃ perovskite active layer deposited by thermal co-evaporation of PbI₂ and MAI. The high-quality co-evaporated perovskite thin films are pinhole-free and uniform over several centimetres, showing large grain sizes, low surface roughness, and a long carrier lifetime. The high-quality perovskite thin films translates to small area PSCs (0.16 cm²) with PCE above 20% and high reproducibility. Large area PSCs with area up to 4 cm² did not show a significant drop in the PCE. Furthermore, the first thermally evaporated mini-modules with an active area larger than 20 cm² achieved the record PCE of 18.13%. [6]

At the same time, looking forward to tandem integration and building-integrated photovoltaics we have also developed coloured semi-transparent PSCs using sputtered indium tin oxide (ITO) as a semi-transparent electrode. The semi-transparent PSCs achieved a consistent PCEs ~16.0% and transparency above 75% in near infrared for all the colors we have developed. Our work represents an important step towards the development of high quality and reproducible large-area perovskite solar cells and mini-modules, the main requirements for the commercialization of the technology.

References:

NREL. Best Research-Cell Efficiency Chart; U.S. Department of Energy; https://www.nrel.gov/pv/cell-efficiency.htm.

A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050.

N. Arora, M. I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S. M. Zakeeruddin, M. Grätzel, Science 2017, 358, 768.

S. Yang, S. Chen, E. Mosconi, Y. Fang, X. Xiao, C. Wang, Y. Zhou, Z. Yu, J. Zhao, Y. Gao, Science 2019, 365, 473.

Y. Wang, T. Wu, J. Barbaud, W. Kong, D. Cui, H. Chen, X. Yang, L. Han, Science 2019 365, 687.

J. Li, H. Wang, X. Y. Chin, H. A. Dewi, K. Vergeer, T. W. Goh, J. W. M. Lim, J. H. Lew, K. P. Loh, C. Soci, T. C. Sum, H. J. Bolink, N. Mathews, S. Mhaisalkar, A. Bruno, Joule 2020, 4, 1035 

CIS is a chalcopyrite thin-film photovoltaic material with the chemical formula CuInSe₂ that serves as a scaffold for the alloying components Ag, Ga, S, in conjunction with alkali metals such as Na, K, Rb, and Cs. In this presentation, the prospects for CIS solar cell research and development, starting from 2020 in Japan, supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI), is introduced. The new NEDO project on CIS solar cells focuses on contributions to the development of light-weight solar modules fabricated by Idemitsu kosan Co., Ltd. (Solar Frontier) using a variety of substrates including metal-foils. Enhancements in CIS solar module performance to extend CIS module use to factory and car roofs, as well as conventional houses and buildings are a key goal.

The talk will include the following topics;

- An introduction to the new NEDO project.

- The road to improvements in the fill factor: (i) recombination in the bulk and at interfaces; (ii) the p-n junction revisited; (iii) device area uniformity; (iv) the physics and chemistry of the CIGS growth process; (v) the modification of CIS photoabsorber layers on different substrate materials; (vi) the CIS/Mo interface.

Antimony selenosulfide, Sb₂(S,Se)₃, is a kind of emerging light harvesting material for solar cell applications. The band gap is tunable in 1.1-1.8 eV with regard to the change in Se/S atomic ratio, it also possesses high extinction coefficient in visible range (～10⁵ cm^-1). In addition, this metal chalcogenide material is stable with moisture an oxygen, thus promising for practical applications. In this presentation, I will introduce our recent research in the development of film fabrication method, where we found that the hydrothermal deposition is able to generate Sb₂(S,Se)₃ film with flat and compact surface morphology, reduced defect and favorable crystal orientation. With that, the solar cell overcoming the 10% efficiency barrier.

In the last few years, the interest and research on new thin film inorganic materials for photovoltaic (PV) applications have largely increased. There are two main motivations for this: i. the necessity to substitute scarce and/or expensive materials currently used in the thin film photovoltaic industry (such as In, Ga or Te); and ii. the necessity of materials with new properties that are required for non-conventional PV applications (flexible, transparent). In this context, kesterite type semiconductors, including Cu₂ZnSn(S,Se)₄ (CZTSSe), Cu₂ZnSnS₄ (CZTS), and Cu₂ZnSnSe₄ (CZTSe) are recognized as one of the most relevant and promising thin film photovoltaic technologies.

In the first part of the presentation, the most relevant progresses achieved for kesterite in the last years will be reviewed, focusing on the recent strategies followed to improve their efficiency, that are mainly based on the partial cationic and anionic substitution including Cu by Ag, Zn by Cd, Mn and Mg, Sn by Ge, extended to the S, Se and Te substitutions as possible anions in the structure. In the second part of the presentation, other relevant emerging photovoltaic materials will be discussed, including oxides, pnictides and halides, giving insights about the most promising ones for different PV applications.

Thin film solar cells are complex structures. The efficiency can only be measured with the full device, where all loss mechanism are combined. We can use photoluminescence to learn about individual loss mechanism, to separate between recombination in the absorber bulk or the interfaces and contacts. We can identify tail states and deep defects that are responsible for radiative and non-radiative losses in the solar cell. And we can predict the diode factor from photoluminesce studies, which has a significant influence on the fill factor of the solar cell. I will summarise the different approaches and show examples from chalcogenide solar cells.

Kesterite-based Cu₂ZnSn(S,Se)₄ semiconductors are emerging as promising materials for low-cost, environment-benign, and high-efficiency thin-film photovoltaics. However, the current state-of-the-art Cu₂ZnSn(S,Se)₄ devices suffer from cation-disordering defects and defect clusters, which generally result in severe potential fluctuation, low minority carrier lifetime, and ultimately unsatisfactory performance. Herein, we report critical growth conditions for obtaining high-quality Cu₂ZnSnSe₄ absorber layers with the formation of detrimental intrinsic defects largely suppressed. By controlling the oxidation states of cations and modifying the local chemical composition, we essentially modify the local chemical environment during the synthesis of kesterite phase, thereby effectively suppressing detrimental intrinsic defects and activating desirable shallow acceptor Cu vacancies. Consequently, we demonstrate a confirmed 12.5% efficiency with high V_OC of 491 mV, which is the new record efficiency of pure-selenide Cu₂ZnSnSe₄ cells with lowest V_OC deficit in the kesterite family by Eg/q-Voc. These encouraging results demonstrate an essential route to overcome the long-standing challenge of defect control in kesterite semiconductors, which may also be generally applicable to other multinary compound semiconductors.

Environmental stability remains one of the greatest barriers to the commercialization of perovskite solar cells (PSCs). The presence of Li-TFSI – a prerequisite dopant for the hole conductor-based Spiro-OMeTAD has been linked to the stability of perovskite devices, e.g., the rapid aggregation and hydration of Li-TFSI upon moisture exposure has been shown to play a key role in the degradation of PSCs.^{1, 2} Here we show that this issue can be tackled by replacing the Li-TFSI with the more hydrophobic alkaline-earth bis(trifluoromethanesulfonyl)imide additives (AEBAs), namely Mg-TFSI₂ and Ca-TFSI₂ owing to the formation of more robust coordination complexes between the TFSI-salts and 4-tert-Butylpyridine. Intriguingly, the presence of AEBAs also improve hole mobilities in Spiro-OMeTAD and energy alignment with adjacent perovskite layer, which ultimately contribute to the favourable carrier extraction at the perovskite/Spiro-OMeTAD interface. Consequently, our PSCs stabilized by the AEBAs yield a champion efficiency of 20.04%, increased from 18.08% for PSCs made with Li-TFSI, while device stability is significantly enhanced.³

We report for the first time the effect of a Cr-doped Cu₂ZnSnS₄ (CZTS) absorber layer on the performance of the CZTS thin film solar cells. Doped films were deposited on Soda-Lime glass substrates through a double step method that includes sulfurization of a sputtered stack of Zn–Cr/Sn/Cu metallic layers. Cr was introduced as a dopant in the Zn layer, and the electro-optical properties of the doped films were investigated as a function of the Cr concentration, showing a large absorption increase for wavelengths above 850 nm. Films with different Cr concentration have been used as absorber layers to investigate their effect on efficiency and performance of the solar cells. Solar cells characterization results show an efficiency increases from 1.86% to 3.96% in cell with 0.04% Cr concentration in the absorber layer. This 113% increase ratio is mainly due to enhancement of the current density assigned to a double step absorption of low energy photons and decrease in deep acceptor type defects. Empirical simulations show that this enhancement is also ascribable to the reduction of the acceptor-type defect density. These findings open the way to highly efficient CZTS solar cells.

The search for the new low cost and high-efficient absorbing material to lead the next Generation of Solar Cells is involving lot of resources and efforts from many cutting-edge research groups and laboratories around the globe. Among different approaches, the power conversion efficiency (PCE) can be increased through the three photon process (TPP) concept, thanks to the two extra sub-bandgap absorptions across the in-gap band (IGB): from the valence band (VB) to the IGB and from the IGB to the conduction band (CB), in addition to VB-CB standard transition. This approach could reach theoretical efficiencies up to 63%, way beyond the Shockley-Queisser limit for single cells.

In this work, solid solutions of group 14 nitrides with spinel structure are selected as host semiconductor material. In this sense, those semiconductors are thermally stable materials very suitable for optoelectronic applications. Concretely, (SnGe)₃N₄ spinel, with a band gap around 2 eV, presents optimal electronic features to host an IB with the desired properties. Doping with different transition metal atoms, an already known technique for this type of materials, we obtain three candidates for novel IGB material with Cu, Cr and Co doping with theoretical efficiencies up to 57%. These efficiencies have been obtained through a deep and complete computational study using Density Functional Theory (DFT), where accurate optical properties have been calculated from previous optimized structures and precise electronic configurations. For that purpose, PBEsol functional, specifically developed for crystal structures of solids, has been used for the optimizations and subsequently high computational cost sc-GW calculations have been carried out to obtain the band structure of the materials. Finally, Bethe-Saltpeter equation as implemented in VASP program has been used to obtain the absorption spectra from the dielectric constants. Final efficiencies have been extracted from those properties using Luque’s methodology, based on detailed-balance model. Our results outstand Cobalt-doped spinel as a very promising material to be used in high-efficient photovoltaic devices.

Organic-inorganic hybrid perovskite solar cells are very promising photovoltaics with a fastly increasing efficiency of over 25%. But the instability of the performance is currently one of the significant technical barriers to their commercialization. Mobile ions migration/accumulation is reported to be the physical mechanism for performance instability, I-V hysteresis, and performance changes during light soaking. An extensive comprehension of how illumination/light-soaking influence perovskites are of fundamental significance to improve the stability of its optoelectronic applications. In this report, a light soaking study on several kinds of perovskites monocrystalline and polycrystalline film will be discussed via temporal-resolved and detection-wavelength selective micro-imaging spectroscopic techniques.[1-5] These works clarify different roles of mobile ion and charge carriers and demonstrate the advantages of the imaging spectroscopy in studying the carrier dynamics of perovskite-based materials under light soaking.

Perovskite photovoltaics (PV) is becoming a competitive technology to CIGS, CdTe and crystalline Si solar cells, especially after addressing its key issues, such as hysteresis [1], stability [2], and scalability [3]. However, the issue of Pb toxicity still remains an obstacle that can hinder potential of perovskite PV. According to the WHO, the blood Pb level for children should not exceed 5 mg/L, which is the amount of Pb contained in only 5x5 mm² of a perovskite absorber layer. Although, Pb-based perovskite PV does not lose its advantages for solar power stations located in non-residential areas, the neurotoxicity of Pb should be considered extremely seriously in the case of building integrated perovskite PV systems. For that reason, the development of non-toxic Pb-free halide semiconductors for photovoltaic applications is highly desirable.

In my presentation I am going to review recent progress in Bi-based photovoltaic rudorffites. In contrast to double-perovskites C₂ABX₆ (C=Cs, MA; A=Ag, Cu; B=Bi, Sb; X=Br, I) consisting of corner shared AX₆ and BX₆ octahedra and hybrid C₃B₂X₉ halides, such as MA₃Bi₂I₉, consisting of isolated face-shared B₂X₉ bioctahedra, the family of rudorffite materials with a general formula of A_aB_bX_x (A=Ag, Cu; B=Bi, Sb; X=Br, I, and x=a+3b) has a structural motif based on edge-shared AX₆ and BX₆. Several members of this family, such as Ag₃BiI₆, Ag₂BiI₅, AgBiI₄, AgBi₂I₇, have been extensively studied for solar cell applications during the last three years [4,5]. This family of materials was named rudorffites, after Walter von Rudorff, who discovered their prototype oxide NaVO₂. Rudorffites feature direct bandgaps in the range of 1.76-1.83 eV corresponding to possible PCE of up to 18% by assuming Voc of 1.2V, typical optical losses and FF for optimized solar cells. It turns out, however, that rudorffite solar cells do not follow the rapid PCE improvement trend that we observed for Pb-based hybrid perovskites. In contrast to electronically clean hybrid perovskites, where native point defects usually do not create mid-gap levels acting as recombination centers, the control of electronically active point defects in rudorffite absorbers is highly important issue as it is in the case of GIGS and CZTS. I am also going to highlight my recent achievements in fabrication of morphologically perfect rudorffite layers through iodination of Ag-Bi bimetallic films and discuss further technological improvements that can help to reveal full potential of rudorffite solar cells.