Understanding Temperature‐Dependent Charge Extraction and Trapping in Perovskite Solar Cells

Understanding the factors that limit the performance of perovskite solar cells (PSCs) can be enriched by detailed temperature (T)‐dependent studies. Based on p‐i‐n type PSCs with prototype methylammonium lead triiodide (MAPbI₃) perovskite absorbers, T‐dependent photovoltaic properties are explored and negative T‐coefficients for the three device parameters (V_OC, J_SC, and FF) are observed within a wide low T‐range, leading to a maximum power conversion efficiency (PCE) of 21.4% with an impressive fill factor (FF) approaching 82% at 220 K. These T‐behaviors are explained by the enhanced interfacial charge transfer, reduced charge trapping with suppressed nonradiative recombination and narrowed optical bandgap at lower T. By comparing the T‐dependent device behaviors based on MAPbI₃ devices containing a PASP passivation layer, enhanced PCE at room temperature is observed but different tendencies showing attenuating T‐dependencies of J_SC and FF, which eventually leads to nearly T‐invariable PCEs. These results indicate that charge extraction with the utilized all‐organic charge transporting layers is not a limiting factor for low‐T device operation, meanwhile the trap passivation layer of choice can play a role in the T‐dependent photovoltaic properties and thus needs to be considered for PSCs operating in a temperature‐variable environment.     

Cation‐Exchange Synthesis of Highly Monodisperse PbS Quantum Dots from ZnS Nanorods for Efficient Infrared Solar Cells

Infrared solar cells that utilize low‐bandgap colloidal quantum dots (QDs) are promising devices to enhance the utilization of solar energy by expanding the harvested photons of common photovoltaics into the infrared region. However, the present synthesis of PbS QDs cannot produce highly efficient infrared solar cells. Here, a general synthesis is developed for low‐bandgap PbS QDs (0.65–1 eV) via cation exchange from ZnS nanorods (NRs). First, ZnS NRs are converted to superlattices with segregated PbS domains within each rod. Then, sulfur precursors are released via the dissolution of the ZnS NRs during the cation exchange, which promotes size focusing of PbS QDs. PbS QDs synthesized through this new method have the advantages of high monodispersity, ease‐of‐size control, in situ passivation of chloride, high stability, and a “clean” surface. Infrared solar cells based on these PbS QDs with different bandgaps are fabricated, using conventional ligand exchange and device structure. All of the devices produced in this manner show excellent performance, showcasing the high quality of the PbS QDs. The highest performance of infrared solar cells is achieved using ≈0.95 eV PbS QDs, exhibiting an efficiency of 10.0% under AM 1.5 solar illumination, a perovskite‐filtered efficiency of 4.2%, and a silicon‐filtered efficiency of 1.1%.     

Coordinatable and High Charge‐Carrier‐Mobility Water‐Soluble Conjugated Copolymers for Effective Aqueous‐Processed Polymer–Nanocrystal Hybrid Solar Cells and OFET Applications

A water‐soluble conjugated polymer (WCP) poly[(3,4‐dibromo‐2,5‐thienylene vinylene)‐co‐(p‐phenylene‐vinylene)] (PBTPV), containing thiophene rings with high charge‐carrier mobility and benzene rings with excellent solubility is designed and prepared through Wessling polymerization. The PBTPV precursor can be easily processed by employing water or alcohols as the solvents, which are clean, environmentally friendly, and non‐toxic compared with the highly toxic organic solvents such as chloroform and chlorobenzene. As a novel photoelectric material, PBTPV presents excellent hole‐transport properties with a carrier mobility of 5 × 10^?4 cm² V^?1 s^?1 measured in an organic field‐effect transistor device. By integrating PBTPV with aqueous CdTe nanocrystals (NCs) to produce the active layer of water‐processed hybrid solar cells, the devices exhibit effective power conversion efficiency up to 3.3%. Moreover, the PBTPV can form strong coordination interactions with the CdTe NCs through the S atoms on the thiophene rings, and effective coordination with other nanoparticles can be reasonably expected.     

Highly Sensitive Temperature Sensor: Ligand‐Treated Ag Nanocrystal Thin Films on PDMS with Thermal Expansion Strategy

Highly sensitive temperature sensors are designed by exploiting the interparticle distance–dependent transport mechanism in nanocrystal (NC) thin films based on a thermal expansion strategy. The effect of ligands on the electronic, thermal, mechanical, and charge transport properties of silver (Ag) NC thin films on thermal expandable substrates of poly(dimethylsiloxane) (PDMS) is investigated. While inorganic ligand‐treated Ag NC thin films exhibit a low temperature coefficient of resistance (TCR), organic ligand‐treated films exhibit extremely high TCR up to 0.5 K^?1, which is the highest TCR exhibited among nanomaterial‐based temperature sensors to the best of the authors' knowledge. Structural and electronic characterizations, as well as finite element method simulation and transport modeling are conducted to determine the origin of this behavior. Finally, an all‐solution based fabrication process is established to build Ag NC‐based sensors and electrodes on PDMS to demonstrate their suitability as low‐cost, high‐performance attachable temperature sensors.     

High Performance PbS Colloidal Quantum Dot Solar Cells by Employing Solution‐Processed CdS Thin Films from a Single‐Source Precursor as the Electron Transport Layer

CdS thin films are a promising electron transport layer in PbS colloidal quantum dot (CQD) photovoltaic devices. Some traditional deposition techniques, such as chemical bath deposition and RF (radio frequency) magnetron sputtering, have been employed to fabricate CdS films and CdS/PbS CQD heterojunction photovoltaic devices. However, their power conversion efficiencies (PCEs) are moderate compared with ZnO/PbS and TiO₂/PbS heterojunction CQD solar cells. Here, efficiencies have been improved substantially by employing solution‐processed CdS thin films from a single‐source precursor. The CdS film is deposited by a straightforward spin‐coating and annealing process, which is a simple, low‐cost, and high‐material‐usage fabrication process compared to chemical bath deposition and RF magnetron sputtering. The best CdS/PbS CQD heterojunction solar cell is fabricated using an optimized deposition and air‐annealing process achieved over 8% PCE, demonstrating the great potential of CdS thin films fabricated by the single‐source precursor for PbS CQDs solar cells.     

Solution‐Processed Metal Oxide Nanocrystals as Carrier Transport Layers in Organic and Perovskite Solar Cells

There has been rapid progress in solution‐processed organic solar cells (OSCs) and perovskite solar cells (PVSCs) toward low‐cost and high‐throughput photovoltaic technology. Carrier (electron and hole) transport layers (CTLs) play a critical role in boosting their efficiency and long‐time stability. Solution‐processed metal oxide nanocrystals (SMONCs) as a promising CTL candidate, featuring robust process conditions, low‐cost, tunable optoelectronic properties, and intrinsic stability, offer unique advantages for realizing cost‐effective, high‐performance, large‐area, and mechanically flexible photovoltaic devices. In this review, the recent development of SMONC‐based CTLs in OSCs and PVSCs is summarized. This paper starts with the discussion of synthesis approaches of SMONCs. Then, a broad range of SMONC‐based CTLs, including hole transport layers and electron transport layers, are reviewed, in which an emphasis is placed on the improvement of the efficiency and device stability. Finally, for the better understanding of the challenges and opportunities on SMONC‐based CTLs, several strategies and perspectives are outlined.     

Molecular‐Level Switching of Polymer/Nanocrystal Non‐Covalent Interactions and Application in Hybrid Solar Cells

Hybrid composites obtained upon blending conjugated polymers and colloidal semiconductor nanocrystals are regarded as attractive photo­active materials for optoelectronic applications. Here it is demonstrated that tailoring nanocrystal surface chemistry permits to control non‐covalent and electronic interactions between organic and inorganic components. The pending moieties of organic ligands at the nanocrystal surface are shown to not merely confer colloidal stability while hindering charge separation and transport, but drastically impact morphology of hybrid composites during formation from blend solutions. The relevance of this approach to photovoltaic applications is demonstrated for composites based on poly(3‐hexylthiophene) and lead sulfide nanocrystals, considered as inadequate until this report, which enable the fabrication of hybrid solar cells displaying a power conversion efficiency that reaches 3%. By investigating (quasi)steady‐state and time‐resolved photo‐induced processes in the nanocomposites and their constituents, it is ascertained that electron transfer occurs at the hybrid interface yielding long‐lived separated charge carriers, whereas interfacial hole transfer appears hindered. Here a reliable alternative aiming to gain control over macroscopic optoelectronic properties of polymer/nanocrystal composites by mediating their non‐covalent interactions via ligands' pending moieties is provided, thus opening new possibilities towards efficient solution‐processed hybrid solar cells.     

Photophysics of Molecular‐Weight‐Induced Losses in Indacenodithienothiophene‐Based Solar Cells

The photovoltaic performance and optoelectronic properties of a donor–acceptor copolymer are reported based on indacenodithienothiophene (IDTT) and 2,3‐bis(3‐(octyloxy)phenyl)quinoxaline moieties (PIDTTQ) as a function of the number‐average molecular weight (M_n). Current–voltage measurements and photoinduced charge carrier extraction by linear increasing voltage (photo‐CELIV) reveal improved charge generation and charge transport properties in these high band gap systems with increasing M_n, while polymers with low molecular weight suffer from diminished charge carrier extraction because of low mobility–lifetime (μτ) product. By combining Fourier‐transform photocurrent spectroscopy (FTPS) with electroluminscence spectroscopy, it is demonstrate that increasing M_n reduces the nonradiative recombination losses. Solar cells based on PIDTTQ with M_n = 58 kD feature a power conversion efficiency of 6.0% and a charge carrier mobility of 2.1 × 10^?4 cm² V^?1 s^?1 when doctor bladed in air, without the need for thermal treatment. This study exhibits the strong correlations between polymer fractionation and its optoelectronics characteristics, which informs the polymer design rules toward highly efficient organic solar cells.     

Enhanced‐Light‐Harvesting Amphiphilic Ruthenium Dye for Efficient Solid‐State Dye‐Sensitized Solar Cells

A ruthenium sensitizer (coded C101, NaRu (4,4′‐bis(5‐hexylthiophen‐2‐yl)‐2,2′‐bipyridine) (4‐carboxylic acid‐4′‐caboxylate‐2,2′‐bipyridine) (NCS)₂) containing a hexylthiophene‐conjugated bipyridyl group as an ancillary ligand is presented for use in solid‐state dye‐sensitized solar cells (SSDSCs). The high molar‐extinction coefficient of this dye is advantageous compared to the widely used Z907 dye, (NaRu (4‐carboxylic acid‐4′‐carboxylate) (4,4′‐dinonyl‐2,2′‐bipyridine) (NCS)₂). In combination with an organic hole‐transporting material (spiro‐MeOTAD, 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine) 9, 9′‐spirobifluorene), the C101 sensitizer exhibits an excellent power‐conversion efficiency of 4.5% under AM 1.5 solar (100 mW cm^?2) irradiation in a SSDSC. From electronic‐absorption, transient‐photovoltage‐decay, and impedance measurements it is inferred that extending the π‐conjugation of spectator ligands induces an enhanced light harvesting and retards the charge recombination, thus favoring the photovoltaic performance of a SSDSC.     

High‐Performance Air‐Processed Polymer–Fullerene Bulk Heterojunction Solar Cells

High photovoltaic device performance is demonstrated in ambient‐air‐processed bulk heterojunction solar cells having an active blend layer of organic poly(3‐hexylthiophene) (P3HT): [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), with power conversion efficiencies as high as 4.1%, which is comparable to state‐of‐the‐art bulk heterojunction devices fabricated in air‐free environments. High‐resolution transmission electron microscopy is combined with detailed analysis of electronic carrier transport in order to quantitatively understand the effects of oxygen exposure and different thermal treatments on electronic conduction through the highly nanostructured active blend network. Improvement in photovoltaic device performance by suitable post‐fabrication thermal processing results from the reduced oxygen charge trap density in the active blend layer and is consistent with a corresponding slight increase in thickness of an ～4 nm aluminum oxide hole‐blocking layer present at the electron‐collecting contact interface.     

Water‐Soluble Polyfluorenes as an Interfacial Layer Leading to Cathode‐Independent High Performance of Organic Solar Cells

Novel poly[(9,9‐bis((6′‐(N,N,N‐trimethylammonium)hexyl)‐2,7‐fluorene)‐alt‐(9,9‐bis(2‐(2‐(2‐methoxyethoxy)ethoxy)ethyl)‐9‐fluorene)) dibromide (WPF‐6‐oxy‐F) and poly[(9,9‐bis((6′‐(N,N,N‐trimethylammonium)hexyl)‐2,7‐fluorene)‐alt‐(9,9‐bis(2‐(2‐methoxyethoxy)ethyl)‐fluorene)] dibromide (WPF‐oxy‐F) compounds are developed and the use of these water‐soluble polymers as an interfacial layer for low‐cost poly(3‐hexylthiophene):phenyl‐C₆₁ butyric acid methyl ester (P3HT:PCBM) organic solar cells (OSCs) is investigated. When WPF‐oxy‐F or WPF‐6‐oxy‐F is simply inserted between the active layer and the cathode as an interfacial dipole layer by spin‐coating water‐soluble polyfluorenes, the open‐circuit voltage (V_oc), fill factor (FF), and power‐conversion efficiency (PCE) of photovoltaic cells with high work‐function metal cathodes, such as Al, Ag, Au, and Cu, dramatically increases. For example, when WPF‐6‐oxy‐F is used with Al, Ag, Au, or Cu, regardless of the work‐function of the metal cathode, the V_oc is 0.64, 0.64, 0.58, and 0.63 V, respectively, approaching the original value of the P3HT:PCBM system because of the formation of large interfacial dipoles through a reduction of the metal work‐function. In particular, introducing WPF‐6‐oxy‐F into a low‐cost Cu cathode dramatically enhanced the device efficiency from 0.8% to 3.36%.     

Achieving 20% Efficiency for Low‐Temperature‐Processed Inverted Perovskite Solar Cells

Low‐temperature‐processed inverted perovskite solar cells (PVSCs) attract increasing attention because they can be fabricated on both rigid and flexible substrates. For these devices, hole‐transporting layers (HTLs) play an important role in achieving efficient and stable inverted PVSCs by adjusting the anodic work function, hole extraction, and interfacial charge recombination. Here, the use of a low‐temperature (≤150 °C) solution‐processed ultrathin film of poly[(9,9‐dioctyl‐fluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐secbutylphenyl) diphenylamine)] (TFB) is reported as an HTL in one‐step‐processed CH₃NH₃PbI₃ (MAPbI₃)‐based inverted PVSCs. The fabricated device exhibits power conversion efficiency (PCE) as high as 20.2% when measured under AM 1.5 G illumination. This PCE makes them one of the MAPbI₃‐based inverted PVSCs that have the highest efficiency reported to date. Moreover, this inverted PVSC also shows good stability, which can retain 90% of its original efficiency after 30 days of storage in ambient air.     

Temperature‐Dependent Charge‐Carrier Dynamics in CH3NH3PbI3 Perovskite Thin Films

The photoluminescence, transmittance, charge‐carrier recombination dynamics, mobility, and diffusion length of CH₃NH₃PbI₃ are investigated in the temperature range from 8 to 370 K. Profound changes in the optoelectronic properties of this prototypical photovoltaic material are observed across the two structural phase transitions occurring at 160 and 310 K. Drude‐like terahertz photoconductivity spectra at all temperatures above 80 K suggest that charge localization effects are absent in this range. The monomolecular charge‐carrier recombination rate generally increases with rising temperature, indicating a mechanism dominated by ionized impurity mediated recombination. Deduced activation energies E_a associated with ionization are found to increase markedly from the room‐temperature tetragonal (E_a ≈ 20 meV) to the higher‐temperature cubic (E_a ≈ 200 meV) phase adopted above 310 K. Conversely, the bimolecular rate constant decreases with rising temperature as charge‐carrier mobility declines, while the Auger rate constant is highly phase specific, suggesting a strong dependence on electronic band structure. The charge‐carrier diffusion length gradually decreases with rising temperature from about 3 μm at ?93 °C to 1.2 μm at 67 °C but remains well above the optical absorption depth in the visible spectrum. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH₃NH₃PbI₃ under typical field conditions.     

Nongeminate Recombination and Charge Transport Limitations in Diketopyrrolopyrrole‐Based Solution‐Processed Small Molecule Solar Cells

Charge transport and nongeminate recombination are investigated in two solution‐processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)‐based donor molecules, mono‐DPP and bis‐DPP, blended with [6,6]‐phenyl‐C71‐butyric acid methyl ester (PCBM). While the bis‐DPP system exhibits a high fill factor (62%) the mono‐DPP system suffers from pronounced voltage dependent losses, which limit both the fill factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current‐voltage characteristics indicate that the mono‐DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most significant factor leading to the difference in fill factor is the comparatively poor hole transport properties in the mono‐DPP system (2 × 10^?5 cm² V^?1 s^?1 versus 34 × 10^?5 cm² V^?1 s^?1). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.     

Identifying a Threshold Impurity Level for Organic Solar Cells: Enhanced First‐Order Recombination Via Well‐Defined PC84BM Traps in Organic Bulk Heterojunction Solar Cells

Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device, including reducing the open circuit voltage, the short circuit current and the fill factor. Steady state studies show a dramatic increase in the trap‐assisted recombination rate when [6,6]‐phenyl C₈₄ butyric acid methyl ester (PC₈₄BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymer poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and the fullerene derivative [6,6]‐phenyl C₆₁ butyric acid methyl ester (PC₆₀BM). The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley–Read–Hall recombination; the latter is dramatically enhanced by the addition of the PC₈₄BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the mechanism of the trap‐induced recombination loss.     

Influence of Hole‐Transport Layers and Donor Materials on Open‐Circuit Voltage and Shape of I–V Curves of Organic Solar Cells

The effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p‐type materials with HOMOs varying between –5.0 and –5.6 eV as hole‐transport layer (HTL) and as donor in vacuum‐evaporated multilayer p‐i‐metal small‐molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift‐diffusion simulations we are well able to reproduce and explain the experimental I–V curves qualitatively. In FHJ solar cells the open‐circuit voltage (V_oc) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, V_oc decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I–V curves in the cases for which V_oc is independent of the HTL.     

Dispersion‐Dominated Photocurrent in Polymer:Fullerene Solar Cells

Organic bulk heterojunction solar cells are often regarded as near‐equilibrium devices, whose kinetics are set by well‐defined charge carrier mobilities, and relaxation in the density of states is commonly ignored or included purely phenomenologically. Here, the motion of photocreated charges is studied experimentally with picosecond time resolution by a combination of time‐resolved optical probing of electric field and photocurrent measurements, and the data are used to define parameters for kinetic Monte Carlo modelling. The results show that charge carrier motion in a prototypical polymer:fullerene solar cell under operational conditions is orders of magnitude faster than would be expected on the basis of corresponding near‐equilibrium mobilities, and is extremely dispersive. There is no unique mobility. The distribution of extraction times of photocreated charges in operating organic solar cells can be experimentally determined from the charge collection transients measured under pulsed excitation. Finally, a remarkable distribution of the photocurrent over energy is found, in which the most relaxed charge carriers in fact counteract the net photocurrent.     

Influence of Phase Segregation on Recombination Dynamics in Organic Bulk‐Heterojunction Solar Cells

The recombination dynamics of charge carriers in organic bulk‐heterojunction (BHJ) solar cells made of the blend system poly(2,5‐bis(3‐dodecylthiophen‐2‐yl)thieno[2,3‐b]thiophene) (pBTCT‐C₁₂):[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) with a donor–acceptor ratio of 1:1 and 1:4 are studied here. The techniques of charge‐carrier extraction by linearly increasing voltage (photo‐CELIV) and, as local probe, time‐resolved microwave conductivity are used. A difference of one order of magnitude is observed between the two blends in the initially extracted charge‐carrier concentration in the photo‐CELIV experiment, which can be assigned to an enhanced geminate recombination that arises through a fine interpenetrating network with isolated phase regions in the 1:1 pBTCT‐C₁₂:PC₆₁BM BHJ solar cells. In contrast, extensive phase segregation in 1:4 blend devices leads to an efficient polaron generation that results in an increased short‐circuit current density of the solar cells. For both studied ratios a bimolecular recombination of polarons is found using the complementary experiments. The charge‐carrier decay order of above two for temperatures below 300 K can be explained on the basis of a release of trapped charges. This mechanism leads to delayed bimolecular recombination processes. The experimental findings can be generalized to all polymer:fullerene blend systems allowing for phase segregation.     

Origin of Radiation‐Induced Degradation in Polymer Solar Cells

Polymer solar cells have been shown to degrade under X‐rays. Here, in situ polymer photovoltaic performance and recombination lifetimes are measured and it is found that charge accumulation is the primary reason for degradation of solar cells. This is affected by the mixing ratio of donor and acceptor in the bulk heterojunction. Both a quantitative understanding and the physical model of the degradation mechanism are presented. Understanding of the degradation mechanism is extended in polymer donor–acceptor bulk heterojunction systems to propose a material combination for making radiation hard diodes that can find important application in fields ranging from memory arrays to organic X‐ray detectors for medical imaging.     

The Origin of the High Voltage in DPM12/P3HT Organic Solar Cells

Organic solar cells made using a blend of DPM12 and P3HT are studied. The results show that higher V_oc can be obtained when using DPM12 in comparison to the usual mono‐substituted PCBM electron acceptor. Moreover, better device performances are also registered when the cells are irradiated with sun‐simulated light of 10–50 mW cm^?2 intensity. Electrochemical and time‐resolved spectroscopic measurements are compared for both devices and a 100‐mV shift in the density of states (DOS) is observed for DPM12/P3HT devices with respect to PCBM/P3HT solar cells and slow polaron‐recombination dynamics are found for the DPM12/P3HT devices. These observations can be directly correlated with the observed increase in V_oc, which is in contrast with previous results that correlated the higher V_oc with different ideality factors obtained using dark‐diode measurements. The origin for the shift in the DOS can be correlated to the crystallinity of the blend that is influenced by the properties of the included fullerene.