Efficient Hybrid Tandem Solar Cells Based on Optical Reinforcement of Colloidal Quantum Dots with Organic Bulk Heterojunctions

While colloidal quantum dot photovoltaic devices (CQDPVs) can achieve a power conversion efficiency (PCE) of ≈12%, their insufficient optical absorption in the near‐infrared (NIR) regime impairs efficient utilization of the full spectrum of visible light. Here, high‐efficiency, solution‐processed, hybrid series, tandem photovoltaic devices are developed featuring CQDs and organic bulk heterojunction (BHJ) photoactive materials for front‐ and back‐cells, respectively. The organic BHJ back‐cell efficiently harvests the transmitted NIR photons from the CQD front‐cell, which reinforces the photon‐to‐current conversion at 350–1000 nm wavelengths. Optimizing the short‐circuit current density balance of each sub‐cell and creating a near ideal series connection using an intermediate layer achieve a PCE (12.82%) that is superior to that of each single‐junction device (11.17% and 11.02% for the CQD and organic BHJ device, respectively). Notably, the PCE of the hybrid tandem device is the highest among the reported CQDPVs, including single‐junction devices and tandem devices. The hybrid tandem device also exhibits almost negligible degradation after air storage for 3 months. This study suggests a potential route to improve the performance of CQDPVs by proper hybridization with NIR‐absorbing photoactive materials.     

Impact of Hole Transport Layer Surface Properties on the Morphology of a Polymer‐Fullerene Bulk Heterojunction

Investigations on the impact of interfacial modification on organic optoelectronic device performance often attribute the improved device performance to the optoelectronic properties of the modifier. A critical assumption of such conclusions is that the organic active layer deposited on top of the modified surface (interface) remains unaltered. Here the validity of this assumption is investigated by examining the impact of substrate surface properties on the morphology of poly(3‐hexylthiophene):1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐[6,6]C₆₁ (P3HT:PCBM) bulk‐heterojunction (BHJ). A set of four nickel oxide and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layers (HTL) with contrasting surface properties and performance in organic photovoltaic (OPV) devices is studied. Differences in vertical composition variation and structural morphologies are observed across the samples, but only in the near‐interface region of <～20 nm. Near‐interface differences in morphology are most closely correlated with surface polarity and surface roughness of the HTL. Surface polarity is more influenced by surface composition than surface roughness and crystal structure. These findings corroborate the previously mentioned conclusions that the differences in device performance observed in solar cells employing these HTLs are dominated by the electronic properties of the HTL/organic photoactive active layer interface and not by unintentional alteration in the BHJ active layer morphology.     

Overcoming Interfacial Losses in Solution‐Processed Organic Multi‐Junction Solar Cells

Organic solar cells are promising in terms of full‐solution‐processing which enables low‐cost and large‐scale fabrication. While single‐junction solar cells have seen a boost in power conversion efficiency (PCE), multi‐junction solar cells are promising to further enhance the PCE. In all‐solution‐processed multi‐junction solar cells, interfacial losses are often encountered between hole‐transporting layer (HTL) and the active layers and therefore greatly limit the application of newly developed high‐performance donor and acceptor materials in multi‐junction solar cells. Here, the authors report on a systematic study of interface losses in both single‐junction and multi‐junction solar cells based on representative polymer donors and HTLs using electron spectroscopy and time‐of‐flight secondary ion mass spectrometry. It is found that a facile mixed HTL containing poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and MoO _x nanoparticles successfully overcomes the interfacial losses in both single‐ and multi‐junction solar cells based on various active layers by reducing interface protonation, promoting better energy‐level alignment, and forming a dense and smooth layer. Solution‐processed single‐junction solar cells are demonstrated to reach the same performance as with evaporated MoO _x (over 7%). Multi‐junction solar cells with polymers containing nitrogen atoms as the first layer and the mixed PEDOT:PSS and MoO _x nanoparticles as hole extraction layer reach fill factor (FF) of over 60%, and PCE of over 8%, while the identical stack with pristine PEDOT:PSS or MoO _x nanoparticles show FF smaller than 50% and PCE less than 5%.     

Molecular Engineering in Hole Transport π‐Conjugated Polymers to Enable High Efficiency Colloidal Quantum Dot Solar Cells

Organic p‐type materials are potential candidates as solution processable hole transport materials (HTMs) for colloidal quantum dot solar cells (CQDSCs) because of their good hole accepting/electron blocking characteristics and synthetic versatility. However, organic HTMs have still demonstrated inferior performance compared to conventional p‐type CQD HTMs. In this work, organic π‐conjugated polymer (π‐CP) based HTMs, which can achieve performance superior to that of state‐of‐the‐art HTM, p‐type CQDs, are developed. The molecular engineering of the π‐CPs alters their optoelectronic properties, and the charge generation and collection in CQDSCs using them are substantially improved. A device using PBDTTPD‐HT achieves power conversion efficiency (PCE) of 11.53% with decent air‐storage stability. This is the highest reported PCE among CQDSCs using organic HTMs, and even higher than the reported best solid‐state ligand exchange‐free CQDSC using pCQD‐HTM. From the viewpoint of device processing, device fabrication does not require any solid‐state ligand exchange step or layer‐by‐layer deposition process, which is favorable for exploiting commercial processing techniques.     

Effect of Photogenerated Dipoles in the Hole Transport Layer on Photovoltaic Performance of Organic–Inorganic Perovskite Solar Cells

Judicious choice of transport layer in organic–inorganic halide perovskite solar cells can be one of the essential parameters in photovoltaic design and fabrication techniques. This article reports the effect of optically generated dipoles in transport layer on the photovoltaic actions in active layer in perovskite solar cells with the architecture of indium tin oxide (ITO)/TiO _x /CH₃NH₃PbI_3–x Cl _x /hole transport layer (HTL)/Au. Here, PTB7‐thieno[3,4‐b]thiophene‐alt‐benzodithiophene and P3HT‐poly(3‐hexylthiophene) are separately used as the HTL with significant and negligible photoinduced dipoles, respectively. Electric field‐induced photoluminescence quenching provides the first‐hand evidence to indicate that the photoinduced dipoles are partially aligned in the amorphous PTB7 layer under the influence of device built‐in field. By monitoring the recombination process through magneto‐photocurrent measurements under device operation condition, it is shown that the photoinduced dipoles in PTB7 layer can decrease the recombination of photogenerated carriers in the active layer in perovskite solar cells. Furthermore, the capacitance measurements suggest that the photoinduced dipoles in PTB7 can decrease charge accumulation at the electrode interface. Therefore, the studies indicate the important role of photoinduced dipoles in the HTL on charge recombination dynamics and provide a fundamental insight on how the polarization in transport layer can influence the device performance in perovskite solar cells.     

High‐Performance Tandem Organic Solar Cells Using HSolar as the Interconnecting Layer

Tandem structure provides a practical way to realize high efficiency organic photovoltaic cells, it can be used to extend the wavelength coverage for light harvesting. The interconnecting layer (ICL) between subcells plays a critical role in the reproducibility and performance of tandem solar cells, yet the processability of the ICL has been a challenge. In this work the fabrication of highly reproducible and efficient tandem solar cells by employing a commercially available material, PEDOT:PSS HTL Solar (HSolar), as the hole transporting material used for the ICL is reported. Comparing with the conventional PEDOT:PSS Al 4083 (c‐PEDOT), HSolar offers a better wettability on the underlying nonfullerene photoactive layers, resulting in better charge extraction properties of the ICL. When FTAZ:IT‐M and PTB7‐Th:IEICO‐4F are used as the subcells, a power conversion efficiency (PCE) of 14.7% is achieved in the tandem solar cell. To validate the processability of these tandem solar cells, three other research groups have successfully fabricated tandem devices using the same recipe and the highest PCE obtained is 16.1%. With further development of donor polymers and device optimization, the device simulation results show that a PCE > 22% can be realized in tandem cells in the near future.     

Evaporation‐Free Nonfullerene Flexible Organic Solar Cell Modules Manufactured by An All‐Solution Process

To ensure laboratory‐to‐industry transfer of next‐generation energy harvesting organic solar cells (OSCs), it is necessary to develop flexible OSC modules that can be produced on a continuous roll‐to‐roll basis and to apply an all‐solution process. In this study, nonfullerene acceptors (NFAs)‐based donor polymer, SMD2, is newly designed and synthesized to continuously fabricate high‐performance flexible OSC modules. Also, multifunctional hole transport layers (HTLs), WO₃/HTL solar bilayer HTLs, are developed and applied via an all‐solution process called “ProcessOne” into inverted structure. SMD2, the donor terpolymer, has a deep highest occupied molecular orbital (HOMO) level and can achieve a power conversion efficiency (PCE) of 11.3% with NFAs without any pre‐/post‐treatment because of its optimal balance between crystallinity and miscibility. Furthermore, the integration of multifunctional HTLs enables the recovery of the drop in open circuit voltage (V_OC) caused by a mismatch in energy levels between the deep HOMO level of the NFAs‐based bulk‐heterojunction layer and the solution‐processed HTLs. Also, the photostability under ultraviolet‐exposure necessary for “ProcessOne” is greatly improved because of the integration of multifunctional HTLs. Consequently, because of the synergistic effects of these approaches, the flexible OSC modules fabricated in an industrial production line have a PCE of 5.25% (P_max = 419.6 mW) on an active area of 80 cm².     

Hot‐Substrate Deposition of Hole‐ and Electron‐Transport Layers for Enhanced Performance in Perovskite Solar Cells

Charge transport layers play an important role in determining the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs). However, it has proven challenging to produce thin and compact charge transport layers via solution processing techniques. Herein, a hot substrate deposition method capable of improving the morphology of high‐coverage hole‐transport layers (HTLs) and electron‐transport layers (ETLs) is reported. PSC devices using HTLs deposited on a hot substrate show improvement in the open‐circuit voltage (V_oc) from 1.041 to 1.070 V and the PCE from 17.00% to 18.01%. The overall device performance is then further enhanced with the hot substrate deposition of ETLs as the V_oc and PCE reach 1.105 V and 19.16%, respectively. The improved performance can be explained by the decreased current leakage and series resistance, which are present in PSCs with rough and discontinuous HTLs and ETLs.     

Efficiency Enhancement of Polymer Solar Cells Based on Poly(3‐hexylthiophene)/Indene‐C70 Bisadduct via Methylthiophene Additive

Photovoltaic performance of polymer solar cells based on poly(3‐hexylthiophene) (P3HT) as the donor and indene‐C₇₀ bisadduct (IC₇₀BA) as the acceptor is improved by adding 3 vol% 3‐methylthiophene (MT) or 3‐hexylthiophene (HT) as processing additives. The results of UV‐vis absorption spectroscopy, X‐ray diffraction analysis and atomic force microscopy indicate that with the MT or HT processing additive, the active layer of the blend of P3HT/IC₇₀BA showed strengthened absorbance, enhanced crystallinity and improved film morphology. The power conversion efficiency (PCE) of the PSCs was improved from 5.80% for the device without the additive to 6.35% for the device with HT additive and to 6.69% with MT additive. The PCE of 6.69% is the top value reported so far for the PSCs based on P3HT.     

Electron‐Beam‐Evaporated Nickel Oxide Hole Transport Layers for Perovskite‐Based Photovoltaics

High‐quality charge carrier transport materials are of key importance for stable and efficient perovskite‐based photovoltaics. This work reports on electron‐beam‐evaporated nickel oxide (NiO_x) layers, resulting in stable power conversion efficiencies (PCEs) of up to 18.5% when integrated into solar cells employing inkjet‐printed perovskite absorbers. By adding oxygen as a process gas and optimizing the layer thickness, transparent and efficient NiO_x hole transport layers (HTLs) are fabricated, exhibiting an average absorptance of only 1%. The versatility of the material is demonstrated for different absorber compositions and deposition techniques. As another highlight of this work, all‐evaporated perovskite solar cells employing an inorganic NiO_x HTL are presented, achieving stable PCEs of up to 15.4%. Along with good PCEs, devices with electron‐beam‐evaporated NiO_x show improved stability under realistic operating conditions with negligible degradation after 40 h of maximum power point tracking at 75 °C. Additionally, a strong improvement in device stability under ultraviolet radiation is found if compared to conventional perovskite solar cell architectures employing other metal oxide charge transport layers (e.g., titanium dioxide). Finally, an all‐evaporated perovskite solar mini‐module with a NiO_x HTL is presented, reaching a PCE of 12.4% on an active device area of 2.3 cm².     

Inorganic and Hybrid Interfacial Materials for Organic and Perovskite Solar Cells

As organic solar cells (OSCs) and perovskite solar cells (PVSCs) move closer to commercialization, further efforts toward optimizing both cell efficiency and stability are needed. As interfaces strongly affect device performance and degradation processes, interfacial engineering by employing various materials as hole transport layers (HTLs) and electron transport layers (ETLs) has been a very active field of research in OSCs and PVSCs. Among them, inorganic materials exhibit significant advantages in promoting device performance due to their excellent charge transporting properties and intrinsic thermal and chemical robustness. In this review, an extensive overview is provided of inorganic semiconductors such as copper‐based ones with emphasis on copper iodide and copper thiocyanate, transition metal chalcogenides, nitrides and carbides as well as hybrid materials based on these inorganic compounds that have been recently employed as HTLs and ETLs in OSCs and PVSCs. Following a short discussion of the main optoelectronic and physical properties that interfacial materials used as HTLs and ETLs should possess, the functionalities of the aforementioned materials as interfacial, charge transport, layers in OSCs and PVSCs are discussed in depth. It is concluded by providing guidelines for further developments that could significantly extend the implementation of these materials in solar cells.     

High‐Efficiency CsPbI2Br Perovskite Solar Cells with Dopant‐Free Poly(3‐hexylthiophene) Hole Transporting Layers

CsPbI₂Br is emerging as a promising all‐inorganic material for perovskite solar cells (PSCs) due to its more stable lattice structure and moisture resistance compared to CsPbI₃, although its device performance is still much behind this counterpart. Herein, a preannealing process is developed and systematically investigated to achieve high‐quality CsPbI₂Br films by regulating the nucleation and crystallization of perovskite. The preannealing temperature and time are specifically optimized for a dopant‐free poly(3‐hexylthiophene) (P3HT)‐based device to target dopant‐induced drastic performance degradation for spiro‐OMeTAD‐based devices. The resulting P3HT‐based device exhibits comparable power conversion efficiency (PCE) to spiro‐OMeTAD‐based devices but much enhanced ambient stability with over 95% PCE after 1300 h. A diphenylamine derivative is introduced as a buffer layer to improve the energy‐level mismatch between CsPbI₂Br and P3HT. A record‐high PCE of 15.50% for dopant‐free P3HT‐based CsPbI₂Br PSCs is achieved by alleviating the open‐circuit voltage loss with the buffer layer. These results demonstrate that the preannealing processing together with a suitable buffer layer are applicable strategies for developing dopant‐free P3HT PSCs with high efficiency and stability.     

Square‐Centimeter‐Sized High‐Efficiency Polymer Solar Cells: How the Processing Atmosphere and Film Quality Influence Performance at Large Scale

Organic solar cells based on two benzodithiophene‐based polymers (PTB7 and PTB7‐Th) processed at square centimeter‐size under inert atmosphere and ambient air, respectively, are investigated. It is demonstrated that the performance of solar cells processed under inert atmosphere is not limited by the upscaling of photoactive layer and the interfacial layers. Thorough morphological and electrical characterizations of optimized layers and corresponding devices reveal that performance losses due to area enlargement are only caused by the sheet resistance of the transparent electrode reducing the efficiency from 9.3% of 7.8% for PTB7‐Th in the condition that both photoactive layer and the interfacial layers are of high layer quality. Air processing of photoactive layer and the interfacial layers into centimeter‐sized solar cells lead to additional, but only slight, losses (<10%) in all photovoltaic parameters, which can be addressed to changes in the electronic properties of both active layer and ZnO layers rather than changes in layer morphology. The demonstrated compatibility of polymer solar cells using solution‐processed photoactive layer and interfacial layers with large area indicates that the introduction of a standard active area of 1 cm² for measuring efficiency of organic record solar cells is feasible. However electric standards for indium tin oxides (ITO) or alternative transparent electrodes need to be developed so that performance of new photovoltaic materials can be compared at square centimeter‐size.     

Rapid Aqueous Spray Fabrication of Robust NiOx: A Simple and Scalable Platform for Efficient Perovskite Solar Cells

Organometal halide perovskites have powerful intrinsic potential to drive next‐generation solar technology, but their insufficient thermomechanical reliability and unproven large‐area manufacturability limit competition with incumbent silicon photovoltaics. This work addresses these limitations by leveraging large‐area processing and robust inorganic hole transport layers (HTLs). Inverted perovskite solar cells utilizing NiO_x HTLs deposited by rapid aqueous spray‐coating that outperform spin‐coated NiO_x and lead to a 5× improvement in the fracture energy (G_c), a primary metric of thermomechanical stability, are presented. The morphology, chemical composition, and optoelectronic properties of the NiO_x films are characterized to understand and optimize compatibility with an archetypal double cation perovskite, Cs_.17FA_.83Pb(Br_.17I_.83)₃. Perovskite solar cells with sprayed NiO_x show higher photovoltaic performance, exhibiting up to 82% fill factor and 17.7% power conversion efficiency (PCE)—the highest PCE reported for inverted cell with scalable charge transport layers—as well as excellent stability under full illumination and after 4000 h aging in inert conditions at room temperature. By utilizing open‐air techniques and aqueous precursors, this combination of robust materials and low‐cost processing provides a platform for scaling perovskite modules with long‐term reliability.     

Highly‐Tunable Nickel Cobalt Oxide as a Low‐Temperature P‐Type Contact in Organic Photovoltaic Devices

We report on the investigation of nickel cobalt oxide (Ni_xCo_3?xO₄) thin films grown by pulsed laser deposition as hole‐transport interlayers (HTL) in organic photovoltaic (OPV) devices. Films of 7 nm thickness were grown under various oxygen deposition pressures (pO₂) in the range of 2–200 mTorr. We explore both bulk and surface properties of these thin films. The workfunction (?) for each of the films was statistically similar (～4.7 eV), regardless of pO₂. There was not a strong dependence of the power conversion efficiency (η) on the conductivities of the HTLs varying between 0.009 ‐ 10 S/cm. The observed differences in OPV efficiencies (ranging from 1.16 to 2.46%) were correlated to the near surface chemical composition of the Ni_xCo_3?xO₄ HTL, as observed by differences in the relative surface hydroxyl concentration. The critical role of the near‐surface composition of the HTL at the HTL/organic interface was further explored by modifying the hydroxyl concentration using an oxygen plasma treatment. This treatment mitigated the impact of surface hydroxyl coverage, demonstrating either identical or increased values for ? and η, regardless of initial pO₂ in the creation of the Ni_xCo_3?xO₄ HTL. To further explore this we also employed a phosphonic acid surface modification agent on the HTL, increasing ? to 5.2 eV producing the best η value of 3.4%, equivalent to the PEDOT:PSS control devices. These results indicate that nickel cobalt oxide is a promising p‐type oxide for carrier‐selective interlayers in organic solar cells; however, for this to be fully realized the specific surface chemistry at the oxide/polymer interface must be controlled to increase ? and optimize device performance.     

Intra‐Molecular Donor–Acceptor Interaction Effects on Charge Dissociation,Charge Transport,and Charge Collection in Bulk‐Heterojunction Organic Solar Cells

This article reports experimental studies on internal charge dissociation, transport, and collection by using magnetic field effects of photocurrent (MFE_PC) and light‐assisted dielectric response (LADR) in highly‐efficient organic solar cells based on photovoltaic polymer PTB2 and PTB4 with intra‐molecular “donor–acceptor” interaction. The MFE_PC at low‐field (< 150 mT) indicates that intra‐molecular “donor‐acceptor” interaction generates charge dissociation in un‐doped PTB2 and PTB4 films, which is similar to that in lightly doped P3HT (Poly(3‐hexylthiophene)) with 5 wt% PCBM (1‐(3‐methyloxycarbonyl)‐propyl‐1‐phenyl (6,6) C₆₁). After PTB2 and PTB4 are mixed with PCBM to form bulk‐heterojunctions, the MFE_PC at high‐field (> 150 mT) reveals that the charge‐transfer complexes formed at PTB2:PCBM and PTB4:PCBM interfaces have much lower binding energies due to stronger electron‐withdrawing abilities, as compared to the P3HT:PCBM device, towards the generation of photocurrent. Furthermore, the light‐assisted dielectric response: LADR indicates that the PTB2:PCBM and PTB4:PCBM solar cells exhibit larger capacitances relative to P3HT:PCBM device under photoexcitation. This reflects that the PTB2:PCBM and PTB4:PCBM bulk heterojunctions have more effective charge transport and collection than the P3HT:PCBM counterpart. As a result, our experimental results indicate that intra‐molecular “donor‐acceptor” interaction plays an important role to enhance charge dissociation, transport, and collection in bulk‐heterojunction organic solar cells.     

Nanofiber‐Based Bulk‐Heterojunction Organic Solar Cells Using Coaxial Electrospinning

Nanofibers consisting of the bulk heterojunction organic photovoltaic (BHJ–OPV) electron donor–electron acceptor pair poly(3‐hexylthiophene):phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) are produced through a coaxial electrospinning process. While P3HT:PCBM blends are not directly electrospinnable, P3HT:PCBM‐containing fibers are produced in a coaxial fashion by utilizing polycaprolactone (PCL) as an electrospinnable sheath material. Pure P3HT:PCBM fibers are easily obtained after electrospinning by selectively removing the PCL sheath with cyclopentanone (average diameter 120 ± 30 nm). These fibers are then incorporated into the active layer of a BHJ–OPV device, which results in improved short‐circuit current densities, fill factors, and power‐conversion efficiencies (PCE) as compared to thin‐film devices of identical chemical composition. The best‐performing fiber‐based devices exhibit a PCE of 4.0%, while the best thin‐film devices have a PCE of 3.2%. This increase in device performance is attributed to the increased in‐plane alignment of P3HT polymer chains on the nanoscale, caused by the electrospun fibers, which leads to increased optical absorption and subsequent exciton generation. This methodology for improving device performance of BHJ–OPVs could also be implemented for other electron donor–electron acceptor systems, as nanofiber formation is largely independent of the PV material.     

Enhanced Efficiency in Plastic Solar Cells via Energy Matched Solution Processed NiOx Interlayers

We show enhanced efficiency and stability of a high performance organic solar cell (OPV) when the work‐function of the hole collecting indium‐tin oxide (ITO) contact, modified with a solution‐processed nickel oxide (NiO_x) hole‐transport layer (HTL), is matched to the ionization potential of the donor material in a bulk‐heterojunction solar cell. Addition of the NiO_x HTL to the hole collecting contact results in a power conversion efficiency (PCE) of 6.7%, which is a 17.3% net increase in performance over the 5.7% PCE achieved with a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL on ITO. The impact of these NiO_x films is evaluated through optical and electronic measurements as well as device modeling. The valence and conduction band energies for the NiO_x HTL are characterized in detail through photoelectron spectroscopy studies while spectroscopic ellipsometry is used to characterize the optical properties. Oxygen plasma treatment of the NiO_x HTL is shown to provide superior contact properties by increasing the ITO/NiO_x contact work‐function by 500 meV. Enhancement of device performance is attributed to reduction of the band edge energy offset at the ITO/NiO_x interface with the poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothidiazole) (PCDTBT):[6,6]‐phenyl‐C61 butyric acid methyl ester PCBM and [6,6]‐phenyl‐C71 butyric acid methyl ester (PC₇₀BM) active layer. A high work‐function hole collecting contact is therefore the appropriate choice for high ionization potential donor materials in order to maximize OPV performance.     

Terthieno[3,2‐b]Thiophene (6T) Based Low Bandgap Fused‐Ring Electron Acceptor for Highly Efficient Solar Cells with a High Short‐Circuit Current Density and Low Open‐Circuit Voltage Loss

A terthieno[3,2‐b]thiophene ( 6T ) based fused‐ring low bandgap electron acceptor, 6TIC , is designed and synthesized for highly efficient nonfullerene solar cells. The chemical, optical, and physical properties, device characteristics, and film morphology of 6TIC are intensively studied. 6TIC shows a narrow bandgap with band edge reaching 905 nm due to the electron‐rich π‐conjugated 6T core and reduced resonance stabilization energy. The rigid, π‐conjugated 6T also offers lower reorganization energy to facilitate very low V_OC loss in the 6TIC system. The analysis of film morphology shows that PTB7‐Th and 6TIC can form crystalline domains and a bicontinuous network. These domains are enlarged when thermal annealing is applied. Consequently, the device based on PTB7‐Th : 6TIC exhibits a high power conversion efficiency (PCE) of 11.07% with a high J_SC > 20 mA cm^?2 and a high V_OC of 0.83 V with a relatively low V_OC loss (≈0.55 V). Moreover, a semitransparent solar cell based on PTB7‐Th : 6TIC exhibits a relatively high PCE (7.62%). The device can have combined high PCE and high J_SC is quite rare for organic solar cells.     

Suppressing Interfacial Dipoles to Minimize Open‐Circuit Voltage Loss in Quantum Dot Photovoltaics

Quantum‐dot (QD) photovoltaics (PVs) offer promise as energy‐conversion devices; however, their open‐circuit‐voltage (V_OC) deficit is excessively large. Previous work has identified factors related to the QD active layer that contribute to V_OC loss, including sub‐bandgap trap states and polydispersity in QD films. This work focuses instead on layer interfaces, and reveals a critical source of V_OC loss: electron leakage at the QD/hole‐transport layer (HTL) interface. Although large‐bandgap organic materials in HTL are potentially suited to minimizing leakage current, dipoles that form at an organic/metal interface impede control over optimal band alignments. To overcome the challenge, a bilayer HTL configuration, which consists of semiconducting alpha‐sexithiophene (α‐6T) and metallic poly(3,4‐ethylenedioxythiphene) polystyrene sulfonate (PEDOT:PSS), is introduced. The introduction of the PEDOT:PSS layer between α‐6T and Au electrode suppresses the formation of undesired interfacial dipoles and a Schottky barrier for holes, and the bilayer HTL provides a high electron barrier of 1.35 eV. Using bilayer HTLs enhances the V_OC by 74 mV without compromising the J_SC compared to conventional MoO₃ control devices, leading to a best power conversion efficiency of 9.2% (>40% improvement relative to relevant controls). Wider applicability of the bilayer strategy is demonstrated by a similar structure based on shallow lowest‐unoccupied‐molecular‐orbital (LUMO) levels.