Understanding Organic Photovoltaic Materials Using Simple Thermal Analysis Methodologies.

Large strides have been made in designing an ever-increasing set of modern organic materials of high functionality and thus, often, of high complexity, including semiconducting polymers, organic ferroelectrics, light-emitting small molecules, and beyond. Here, we review how broadly applied thermal analysis methodologies, especially differential scanning calorimetry, can be utilized to provide unique information on the assembly and solid-state structure of this extensive class of materials, as well as the phase behavior of intrinsically intricate multicomponent systems. Indeed, highly relevant insights can be gained that are useful, e.g., for further materials-discovery activities and the establishment of reliable processing protocols, in particular if combined with X-ray diffraction techniques, spectroscopic tools, and scanning electron microscopy enabled by vapor-phase infiltration staining. We, hence, illustrate that insights far richer than simple melting point- and glass-transition identification can be obtained with differential scanning calorimetry, rendering it a critical methodology to understand complex matter, including functional macromolecules and blends.

Ruddlesden-Popper Perovskite Nanocrystals as Interface Modification Layer for Efficient Perovskite Solar Cells.

Perovskite nanocrystals are advantageous for interfacial passivation of perovskite solar cells (PSCs), but the insulating long alkyl chain surface ligands impede the charge transfer, while the conventional ligand exchange would possibly introduce surface defects to the nanocrystals. In this work, we reported novel in situ modification of CsPbBr3 nanocrystals using a short chain conjugated molecule 2-methoxyphenylethylammonium iodide (2-MeO-PEAI) for interfacial passivation of PSCs. Transmission electron microscopy studies with atomic resolution unveil the transformation from cubic CsPbBr3 to Ruddlesden-Popper phase (RPP) nanocrystals due to halogen exchange. Synergic passivation by the RPP nanocrystals and 2-MeO-PEA+ has led to suppressed interface defects and enhanced charge carrier transport. Consequently, PSCs with in situ modified RPP nanocrystals achieved a champion power conversion efficiency of 24.39%, along with an improvement in stability. This work brings insights into the microstructural evolution of perovskite nanocrystals, providing a novel and feasible approach for interfacial passivation of PSCs.

Efficient Homojunction Tin Perovskite Solar Cells Enabled by Gradient Germanium Doping.

P-type self-doping is known to hamper tin-based perovskites for developing high-performance solar cells by increasing the background current density and carrier recombination processes. In this work, we propose a gradient homojunction structure with germanium doping that generates an internal electric field across the perovskite film to deplete the charge carriers. This structure reduces the dark current density of perovskite by over 2 orders of magnitude and trap density by an order of magnitude. The resultant tin-based perovskite solar cells exhibit a higher power conversion efficiency of 13.3% and excellent stability, maintaining 95% and 85% of their initial efficiencies after 250 min of continuous illumination and 3800 h of storage, respectively. We reveal the homojunction formation mechanism using density functional theory calculations and molecular level characterizations. Our work provides a reliable strategy for controlling the spatial energy levels in tin perovskite films and offers insights into designing intriguing lead-free perovskite optoelectronics.

Chemisorption-Induced Robust and Homogeneous Tungsten Disulfide Interlayer Enables Stable PEDOT-Free Organic Solar Cells with Over 19% Efficiency.

Construction of a high-quality charge transport layer (CTL) with intimate contact with the substrate via tailored interface engineering is crucial to increase the overall charge transfer kinetics and stability for a bulk-heterojunction (BHJ) organic solar cell (OSC). Here, we demonstrate a surface chemistry strategy to achieve a homogeneous composite hole transport layer (C-HTL) with robust substrate contact by self-assembling two-dimensional tungsten disulfide (WS2) nanosheets on a thin molybdenum oxide (MoO3) film-evaporated indium tin oxide (ITO) substrate. It is found that over such a well-defined C-HTL, WS2 is homogeneously tethered on the ITO/MoO3 substrate stemming from the strong electronic coupling interaction between the building blocks, which enables a favorable interfacial configuration in terms of uniformity. As a result, the D18:L8-BO-based OSC with C-HTL exhibits a power conversion efficiency (PCE) of 19.23%, an 11% improvement over the WS2-based control device, and the highest efficiency among single-junction PEDOT-free binary BHJ OSCs.

Single Crystal Seed Induced Epitaxial Growth Stabilizes α-FAPbI3 in Perovskite Solar Cells.

FAPbI3 stands out as an ideal candidate for the photoabsorbing layer of perovskite solar cells (PSCs), showcasing outstanding photovoltaic properties. Nonetheless, stabilizing photoactive α-FAPbI3 remains a challenge due to the lower formation energy of the competitive photoinactive δ-phase. In this study, we employ tetraethylphosphonium lead tribromide (TEPPbBr3) single crystals as templates for the epitaxial growth of PbI2. The strategic use of TEPPbBr3 optimizes the evolution of intermediates and the crystallization kinetics of perovskites, leading to high-quality and phase-stable α-FAPbI3 films. The TEPPbBr3-modified perovskite exhibits optimized carrier dynamics, yielding a champion efficiency of 25.13% with a small voltage loss of 0.34 V. Furthermore, the target device maintains 90% of its initial PCE under maximum power point (MPP) tracking over 1000 h. This work establishes a promising pathway through single crystal seed based epitaxial growth for achieving satisfactory crystallization regulation and phase stabilization of α-FAPbI3 perovskites toward high-efficiency and stable PSCs.

Orbital Interactions in 2D Dion-Jacobson Perovskites Using Oligothiophene-Based Semiconductor Spacers Enable Efficient Solar Cells.

2D Dion-Jacobson (DJ) perovskites have emerged as promising photovoltaic materials, but the insulating organic spacer has hindered the efficient charge transport. Herein, we successfully synthesized a terthiophene-based semiconductor spacer, namely, 3ThDMA, for 2D DJ perovskite. An interesting finding is that the energy levels of 3ThDMA extensively overlap with the inorganic components and directly contribute to the band formation of (3ThDMA)PbI4, leading to enhanced charge transport across the organic spacer layers, whereas no such orbital interactions were found in (UDA)PbI4, a DJ perovskite based on 1,11-undecanediaminum (UDA). The devices based on (3ThDMA)MAn-1PbnI3n+1 (nominal n = 5) obtained a champion efficiency of 15.25%, which is a record efficiency for 2D DJ perovskite solar cells using long-conjugated spacers (conjugated rings ≥ 3) and a 22.60% efficiency for 3ThDMA-treated 3D PSCs. Our findings provide an important insight into understanding the orbital interactions in 2D DJ perovskite using an organic semiconductor spacer for efficient solar cells.

Chemical Bath Deposited Antimony Oxide Thin Films for Efficient Perovskite Solar Cells.

The metal oxide electron transport layers (ETLs) of n-i-p perovskite solar cells (PSCs) are dominated by TiO2 and SnO2, while the efficacy of the other metal oxide ETLs still lags far behind. Herein, an emerging, economical, and environmentally friendly metal oxide, antimony oxide (Sb2Ox, x = 2.17), prepared by chemical bath deposition is reported as an alternative ETL for PSCs. The deposited Sb2Ox film is amorphous and very thin (∼10 nm) but conformal on rough fluorine-doped tin oxide substrates, showing matched energy levels, efficient electron extraction, and then reduced nonradiative recombination in PSCs. The champion PSC based on the Sb2Ox ETL delivers an impressive power conversion efficiency of 24.7% under one sun illumination, which represents the state-of-the-art performance of all metal oxide ETL-based PSCs. Additionally, the Sb2Ox-based devices show improved operational and thermal stability compared to their SnO2-based counterparts. Armed with these findings, we believe this work offers an optional ETL for perovskites-based optoelectronic devices.

Modulating Nucleation and Crystal Growth of Tin Perovskite Films for Efficient Solar Cells.

The performance of tin halide perovskite solar cells (PSCs) has been severely limited by the rapid crystallization of tin perovskites, which usually leads to an undesirable film quality. In this work, we tackle this issue by regulating the nucleation and crystal growth of tin perovskite films using a small Lewis base additive, urea. The urea-SnI2 interaction facilitates the formation of larger and more uniform clusters, thus accelerating the nucleation process. Additionally, the crystal growth process is extended, resulting in a high-quality tin perovskite film with compact morphology, increased crystallinity, and reduced defects. Consequently, the efficiency of tin PSCs is significantly increased from 10.42% to 14.22%. This work highlights the importance of manipulating the nucleation and crystal growth of tin perovskites to realize efficient tin PSCs.

Application of Bifacial Semitransparent CuInSe2 Absorber to the Bottom Cell in Bifacial Semitransparent Perovskite/CuInSe2 Tandem Solar Cell for Albedo Environment.

Although various types of bifacial solar cells exist, few studies have been conducted on bifacial semitransparent CuInSe2 solar cells (BS-CISe SCs) despite the attractive potential in power generation from both sides in an albedo environment. The optimized BS-CISe SCs with 300 and 800 nm-thick absorber via a streamlined single-stage co-evaporation process exhibit a power conversion efficiency (PCE) of 6.32% and 10.6%, respectively. When double-sided total 2.0 sun illumination is assumed in an albedo environment, the bifacial power generation densities (BPGD) of them increases to 9.41% and 13.9%. Four-terminal bifacial semitransparent tandem solar cells (4T-BST SCs) are fabricated to increase the BPGD by mechanically stacking a BS-perovskite (PVK) top cell on top of a BS-CISe bottom cell with the 300 and 800 nm-thick absorber layers. When summed up, the best top and bottom cell PCEs of the 4T-BST SC with 300 and 800 nm-thick BS-CISe SC are 18.8% and 21.1%, respectively. However, the practical BPGD values of the 4T-BST SC under total 2 sun illumination are interestingly 23.4% and 24.4%, respectively. This is because the BS-CISe bottom cell's thickness affects how much rear-side illumination is transmitted to the BS-PVK top cell, increasing its current density and BPGD.

A Dual-Polythiophene Blending Strategy to Reduce the Efficiency-Stability-Cost Gap of Solar Cells.

Benefiting from the photovoltaic material innovation and delicate device optimization, high-efficiency solar cells employing polymeric materials are thriving. Reducing the gap of cost, efficiency, and stability is the critical challenge faced by the emerging solar cells such as organics, quantum dots and perovskites. Poly(3-alkylthiophene) demonstrates great potential in organic solar cells and quantum dot solar cells as the active layer or the hole transport layer due to its large scalability, excellent photoelectric performance, and favorable hydrophobicity. The present low efficiency and insufficient stability, restrict its commercial application. In this work, a facile strategy of blending two simple polythiophenes is put forward to manipulate the film microstructure and enhance the device efficiency and thermal stability of solar cells. The introduction of P3PT can improve the power conversion efficiency (PCE) of a benchmark cost-effective blend P3HT:O-IDTBR to 7.41%, and the developed ternary solar cells also exhibit increased thermal stability. More strikingly, the quantum dot solar cells with the dual-polythiophene hole transport layer achieve the highest PCE of 10.51%, which is among the topmost efficiencies for quantum dots/polythiophene solar cells. Together, this work provides an effective route to simultaneously optimize the device efficiency and thermal stability of solar cells.

A Deep Dive into Cu2ZnSnS4 (CZTS) Solar Cells: A Review of Exploring Roadblocks, Breakthroughs, and Shaping the Future.

Renewable energy is crucial for sustainable future, and Cu2ZnSnS4 (CZTS) based solar cells shine as a beacon of hope. CZTS, composed of abundant, low-cost, and non-toxic elements, shares similarities with Cu(In,Ga)Se2 (CIGS). However, despite its promise and appealing properties for solar cells, CZTS-based solar cells faces performance challenges owing to inherent issues with CZTS material, and conventional substrate structure complexities. This review critically examines these roadblocks, explores ongoing efforts and breakthroughs, providing insight into the evolving landscape of CZTS-based solar cells research. Furthermore, as an optimistic turn in the field, the review first highlights the crucial need to transition to a superstrate structure for CZTS-based single junction devices, and summarizes the substantial progress made in this direction. Subsequently, dive into the discussion about the fascinating realm of CZTS-based tandem devices, providing an overview of the existing literature as well as outlining the possible potential strategies for enhancing the efficiency of such devices. Finally, the review provides a useful outlook that outlines the priorities for future research and suggesting where efforts should concentrate to shape the future of CZTS-based solar cells.

Ionic Liquid Modified Polymer Intermediate Layer for Improved Charge Extraction toward Efficient and Stable Perovskite/Silicon Tandem Solar Cells.

Monolithic perovskite/silicon tandem solar cells have been attracted much attention in recent years. Despite their high performances, the stability issue of perovskite-based devices is recognized as one of the key challenges to realize industrial application. When comes to the perovskite top subcell, the interface between perovskite and electron transporting layers (usually C60) significantly affects the device efficiency as well as the stability due to their poor adhesion. Here, different from the conventional interfacial passivation using metal fluorides, a hybrid intermediate layer is proposed-PMMA functionalized with ionic liquid (IL)-is introduced at the perovskite/C60 interface. The application of PMMA essentially improves the interfacial stability due to its strong hydrophobicity, while adding IL relieves the charge accumulation between PMMA and the perovskite. Thus, an optimal wide-bandgap perovskite solar cells achieves power conversion efficiency of 20.62%. These cells are further integrated as top subcells with silicon bottom cells in a monolithic tandem structure, presenting an optimized PCE up to 27.51%. More importantly, such monolithic perovskite/silicon cells exhibit superior stability by maintaining 90% of initial efficiency after 1200 h under continuous illumination.

Ultra-Stable ITO-Free Organic Solar Cells and Modules Processed from Non-Halogenated Solvents under Indoor Illumination.

Organic Photovoltaics (OPV) is a very promising technology to harvest artificial illumination and power smart devices of the Internet of Things (IoT). Efficiencies as high as 30.2% have been reported for OPVs under warm white light-emitting diode (LED) light. This is due to the narrow spectrum of indoor light, which leads to an optimal bandgap of ≈1.9 eV. Under full sunlight, OPV devices often suffer from poor stability compared to the established inorganic PV technologies such as crystalline silicon. This study focuses on a potentially very cost-effective Indium Tin Oxide (ITO) free cell stack with absorber materials processed from non-halogenated solvents. These organic solar cells and modules with efficiencies up to 21% can already achieve remarkable stabilities under typical indoor illumination. Aging under 50,000 lux LED lighting leads to very little degradation after more than 11 000 h. This light dose corresponds to more than 110 years under 500 lux. For modules encapsulated with a flexible barrier, extrapolated lifetimes of more than 41 years are achieved. This shows that OPV is mature for the specific application under indoor illumination. Due to the large number of potential organic semiconducting materials, further efficiency increase can be expected.

Passivating Grain Boundaries via Graphene Additive for Efficient Kesterite Solar Cells.

Grain boundaries (GBs)-triggered severe non-radiative recombination is recently recognized as the main culprits for carrier loss in polycrystalline kesterite photovoltaic devices. Accordingly, further optimization of kesterite-based thin film solar cells critically depends on passivating the grain interfaces of polycrystalline Cu2 ZnSn(S,Se)4 (CZTSSe) thin films. Herein, 2D material of graphene is first chosen as a passivator to improve the detrimental GBs. By adding graphene dispersion to the CZTSSe precursor solution, single-layer graphene is successfully introduced into the GBs of CZTSSe absorber. Due to the high carrier mobility and electrical conductivity of graphene, GBs in the CZTSSe films are transforming into electrically benign and do not act as high recombination sites for carrier. Consequently, benefitting from the significant passivation effect of GBs, the use of 0.05 wt% graphene additives increases the efficiency of CZTSSe solar cells from 10.40% to 12.90%, one of the highest for this type of cells. These results demonstrate a new route to further increase kesterite-based solar cell efficiency by additive engineering.

Growth of BiSBr Microsheet Arrays for Enhanced Photovoltaics Performance.

In this study, single-crystalline BiSBr is synthesized using a solution-based approach and conducted a systematic characterization of its photoelectric properties and photovoltaic performances. UV photoelectron spectroscopy and density functional theory (DFT) calculations reveal that BiSBr is an indirect p-type semiconductor, characterized by distinct positions and compositions of the valence band maximum and conduction band minimum. The BiSBr single crystal microrod features a significant electrical conductivity of 14 800 S m-1 along the c-axis, denoting minimal carrier resistance in this direction. For photovoltaic performance assessment, the authors successfully fabricated two homogeneous BiSBr films on TiO2 porous substrates: A microsheet array film via physical vapor deposition (PVD) and solvothermal treatment, and a BiSBr microsheet film via PVD and thermal treatment. The solar cell, comprising a BiSBr microsheet array film with an architecture of fluorine-doped tin oxide FTO/TiO2/BiSBr/(I3 -/I-)/Pt, demonstrated a power conversation efficiency of 1.40%, ≈11 times that of BiSBr microsheet film counterpart. These preliminary results underscore the potential of BiSBr microsheet arrays, producible through low-cost solution processes, as adept light absorbers, enhancing photovoltaic efficiency through effective light scattering and promoting efficient electron-hole separation and transport.

Controlling Nucleation and Crystallization of CsPbI3 Perovskites for Efficient Inverted Solar Cells.

The unfavorable morphology and high crystallization temperature (Tc) of inorganic perovskites pose a significant challenge to their widespread application in photovoltaics. In this study, an effective approach is proposed to enhance the morphology of cesium lead triiodide (CsPbI3) while lowering its Tc. By introducing dimethylammonium acetate into the perovskite precursor solution, a rapid nucleation stage is facilitated, and significantly enhances the crystal growth of the intermediate phase at low annealing temperatures, followed by a slow crystal growth stage at higher annealing temperatures. This results in a uniform and dense morphology in CsPbI3 perovskite films with enhanced crystallinity, simultaneously reducing the Tc from 200 to 150 °C. Applying this approach in positive-intrinsic-negative (p-i-n) inverted cells yields a high power conversion efficiency of 19.23%. Importantly, these cells exhibit significantly enhanced stability, even under stress at 85 °C.

Efficient Inverted Perovskite Photovoltaics Through Surface State Manipulation.

Inverted perovskite solar cells (PSCs) are considered as the most promising avenue for the commercialization of PSCs due to their potential inherent stability. However, suboptimal interface contacts between electron transport layer (ETL) (such as C60) and the perovskite absorbing layer within inverted PSCs always result in reduced efficiency and poor stability. Herein, a surface state manipulation strategy has been developed by employing a highly electronegative 4-fluorophenethylamine hydrochloride (p-F-PEACl) to effectively address the issue of poor interface contacts in the inverted PSCs. The p-F-PEACl demonstrates a robust interaction with perovskite film through bonding of amino group and Cl- with I- and Pb2+ ions in the perovskite, respectively. As such, the surface defects of perovskite film can be significantly reduced, leading to suppressed non-radiative recombination. Moreover, p-F-PEACl also plays a dual role in enhancing the surface potential and improving energy-level alignment at the interfaces between the perovskite and C60 carrier transport layer, which directly contributes to efficient charge extraction. Finally, the open-circuit voltage (Voc) of devices increases from 1.104 V to 1.157 V, leading to an overall efficiency improvement from 22.34% to 24.78%. Furthermore, the p-F-PEACl-treated PSCs also display excellent stability.

Minimizing Voltage Losses via Synergistically Reducing Hetero-Interface Energy Offset for High Efficiency Perovskite Solar Cells.

The open circuit voltage (VOC) losses at multiple interfaces within perovskite solar cells (PSCs) limit the improvements in power conversion efficiency (PCE). Herein, a tailored strategy is proposed to reduce the energy offset at both hetero-interfaces within PSCs to decrease the VOC losses. For the interface of perovskite and electron transport layer where exists a mass of defects, it uses the pyromellitic acid to serve as a molecular bridge, which reduces non-radiative recombination and energy level offset. For the interface of perovskite and hole transport layer, which includes a passivator of PEAI, the detrimental effect (negative shift of work function) of PEAI passivation and optimizing the interface energy level alignment are neutralized by incorporating (2-(4-(bis(4-methoxyphenyl)amino)phenyl)-1-cyanovinyl)phosphonic acid. Owing to synergistically reduced hetero-interface energy offset, the PSCs achieve a PCE of 25.13%, and the VOC is increased from 1.134 to 1.174 V. In addition, the resulting PSCs possess enhanced stability, the unencapsulated PSCs can maintain ≈96% and ≈97% of their initial PCE after 2000 h of aging under ambient conditions and 210 h under operation conditions.

Anti-Solvent-Free Fabrication of Stable FA0.9Cs0.1PbI3 Perovskite Solar Cells with Efficiency Exceeding 24.0% through a Naphthalene-Based Passivator.

The anti-solvent-free fabrication of high-efficiency perovskite solar cells (PSCs) holds immense significance for the transition from laboratory-scale to large-scale commercial applications. However, the device performance is severely hindered by the increased occurrence of surface defects resulting from the lack of control over nucleation and crystallization of perovskite using anti-solvent methods. In this study, 2-(naphthalen-2-yl)ethylamine hydriodide (NEAI) is employed as the surface passivator for perovskite films without using any anti-solvent. Naphthalene demonstrates strong π-π conjugation, which aids in the efficient extraction of charge carriers. Additionally, the naphthalene-ring moieties form a tight attachment to the perovskite surface. After NEAI treatment, FA and I vacancies are selectively occupied by NEA+ and I- in NEAI respectively, thus effectively passivating the surface defects and isolating the surface from moisture. Ultimately, the optimized NEAI-treated device achieves a promising power conversion efficiency (PCE) of 24.19% (with a certified efficiency of 23.94%), featuring a high fill factor of 83.53%. It stands out as one of the reported high PCEs achieved for PSCs using the spin-coating technique without the need for any anti-solvent so far. Furthermore, the NEAI-treated device can maintain ≈87% of its initial PCE after 2000 h in ambient air with a relative humidity of 30% ± 5%.

Electrostatic Potential Design of Solid Additives for Enhanced Molecular Order of Polymer Donor in Efficient Organic Solar Cells.

Polymeric semiconducting materials struggle to achieve fast charge mobility due to low structural order. In this work, five 1H-indene-1,3(2H)dione-benzene structured halogenated solid additives namely INB-5F, INB-3F, INB-1F, INB-1Cl, and INB-1Br with gradually varied electrostatic potential are designed and utilized to regulate the structural order of polymer donor PM6. Molecular dynamics simulations demonstrate that although the dione unit of these additives tends to adsorb on the backbone of PM6, the reduced electrostatic potential of the halogen-substituted benzene can shift the benzene interacting site from alkyl side chains to the conjugated backbone of PM6, not only leading to enhanced π-π stacking in out-of-plane but also arising new π-π stacking in in-plane together with the appearance of multiple backbone stacking in out-of-plane, consequent to the co-existence of face-on and edge-on molecular orientations. This molecular packing transformation further translates to enhanced charge transport and suppressed carrier recombination in their photovoltaics, with a maximum power conversion efficiency of 19.4% received in PM6/L8-BO layer-by-layer deposited organic solar cells.