Open‐Circuit Voltage and Effective Gap of Organic Solar Cells

The open‐circuit voltage (V_OC) of an organic solar cell is limited by the donor‐acceptor material system. The effective gap E_g^eff between the electron affinity of the acceptor and the ionization potential of the donor is usually regarded as the upper limit for V_OC, which is only reached for T → 0 K. This relation is confirmed for a number of small‐molecule bulk heterojunction p‐i‐n type solar cells by varying the temperature and illumination intensity. With high precision, the low temperature limit of V_OC is identical to E_g^eff. Furthermore, the influence of the hole transport material in a p‐doped hole transport layer and the donor‐acceptor mixing ratio on this limit V₀ is found to be negligible. Varying the active material system, the quantitative relation between V₀ and E_g^eff is found to be identity. A comparison of V₀ in a series of nine different donor‐acceptor material combinations opens a pathway to quantitatively determine the ionization potential of a donor material or the electron affinity of an acceptor material.     

Organic Dye Design Tools for Efficient Photocurrent Generation in Dye‐Sensitized Solar Cells: Exciton Binding Energy and Electron Acceptors

The relationship between the exciton binding energies of several pure organic dyes and their chemical structures is explored using density functional theory calculations in order to optimize the molecular design in terms of the light‐to‐electric energy‐conversion efficiency in dye‐sensitized solar cell devices. Comparing calculations with measurements reveals that the exciton binding energy and quantum yield are inversely correlated, implying that dyes with lower exciton binding energy produce electric current from the absorbed photons more efficiently. When a strong electron‐accepting moiety is inserted in the middle of the dye framework, the light‐to‐electric energy‐conversion behavior significantly deteriorates. As verified by electronic‐structure calculations, this is likely due to electron localization near the electron‐deficient group. The combined computational and experimental design approach provides insight into the functioning of organic photosensitizing dyes for solar‐cell applications. This is exemplified by the development of a novel, all‐organic dye (EB‐01) exhibiting a power conversion efficiency of over 9%.     

The Impact of Polymer Regioregularity on Charge Transport and Efficiency of P3HT:PCBM Photovoltaic Devices

The charge transport in pristine poly(3‐hexylthiophene) (P3HT) films and in photovoltaic blends of P3HT with [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) is investigated to study the influence of charge‐carrier transport on photovoltaic efficiency. The field‐ and temperature dependence of the charge‐carrier mobility in P3HT of three different regioregularities, namely, regiorandom, regioregular with medium regioregularity, and regioregular with very high regioregularity are investigated by the time‐of‐flight technique. While medium and very high regioregularity polymers show the typical absorption features of ordered lamellar structures of P3HT in the solid state even without previous annealing, films of regiorandom P3HT are very disordered as indicated by their broad and featureless absorption. This structural difference in the solid state coincides with partially non‐dispersive transport and hole mobilities µ_h of around 10^?4 and 10^?5 cm² V^?1 s^?1 for the high and medium regioregularity P3HT, respectively, and a slow and dispersive charge transport for the regiorandom P3HT. Upon blending the regioregular polymers with PCBM, the hole mobilities are typically reduced by one order of magnitude, but they do not significantly change upon additional post‐spincasting annealing. Only in the case of P3HT with high regioregularity are the electron mobilities similar to the hole mobilities and the charge transport is, thus, balanced. Nonetheless, devices prepared from both materials exhibit similar power conversion efficiencies of 2.5%, indicating that very high regioregularity may not substantially improve order and charge‐carrier transport in P3HT:PCBM and does not lead to significant improvements in the power‐conversion efficiency of photovoltaic devices.     

Temperature‐Resolved Local and Macroscopic Charge Carrier Transport in Thin P3HT Layers

Previous investigations of the field‐effect mobility in poly(3‐hexylthiophene) (P3HT) layers revealed a strong dependence on molecular weight (MW), which was shown to be closely related to layer morphology. Here, charge carrier mobilities of two P3HT MW fractions (medium‐MW: M_n = 7 200 g mol^?1; high‐MW: M_n = 27 000 g mol^?1) are probed as a function of temperature at a local and a macroscopic length scale, using pulse‐radiolysis time‐resolved microwave conductivity (PR‐TRMC) and organic field‐effect transistor measurements, respectively. In contrast to the macroscopic transport properties, the local intra‐grain mobility depends only weakly on MW (being in the order of 10^?2 cm² V^?1 s^?1) and being thermally activated below the melting temperature for both fractions. The striking differences of charge transport at both length scales are related to the heterogeneity of the layer morphology. The quantitative analysis of temperature‐dependent UV/Vis absorption spectra according to a model of F. C. Spano reveals that a substantial amount of disordered material is present in these P3HT layers. Moreover, the analysis predicts that aggregates in medium‐MW P3HT undergo a “pre‐melting” significantly below the actual melting temperature. The results suggest that macroscopic charge transport in samples of short‐chain P3HT is strongly inhibited by the presence of disordered domains, while in high‐MW P3HT the low‐mobility disordered zones are bridged via inter‐crystalline molecular connections.     

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.     

NiOX/MoO3 Bi‐Layers as Efficient Hole Extraction Contacts in Organic Solar Cells

The electronic structure of a bi‐layer hole extraction contact consisting of nickel oxide (NiO_x) and molybdenum trioxide (MoO₃) is determined via ultraviolet and X‐ray photoemission spectroscopy. The bi‐layer presents ideal energetics for the extraction of holes and suppression of carrier recombination at the interface. The application of the NiO_x/MoO₃ bi‐layer as the anode of organic bulk heterojunction solar cells based on PCDTBT/PC₇₁BM leads to improved device performance, which is explained by an intricate charge transfer process across the interface.     

Imaging the Electric Potential within Organic Solar Cells

The charge transport in organic solar cells is investigated by surface potential measurements via scanning Kelvin probe microscopy. Access to the solar cell's cross‐section is gained by milling holes with a focused ion beam which enables the direct scan along the charge transport path. In a study of poly(3‐hexylthiophene):1‐(3‐methoxycarbonyl)propyl‐1‐phenyl[6,6]C61 (P3HT:PCBM) bulk heterojunction solar cells, the open circuit voltage is built up at the top contact. A comparison of the potential distribution within normal and inverted solar cells under operation exhibits strongly different behaviors, which can be assigned to a difference in interface properties.     

Mechanism for Efficient Photoinduced Charge Separation at Disordered Organic Heterointerfaces

Despite the poor screening of the Coulomb potential in organic semiconductors, excitons can dissociate efficiently into free charges at a donor–acceptor heterojunction, leading to application in organic solar cells. A kinetic Monte Carlo model that explains this high efficiency as a two‐step process is presented. Driven by the band offset between donor and acceptor, one of the carriers first hops across the interface, forming a charge transfer (CT) complex. Since the electron and hole forming the CT complex have typically not relaxed within the disorder‐broadened density of states (DOS), their remaining binding energy can be overcome by further relaxation in the DOS. The model only contains parameters that are determined from independent measurements and predicts dissociation yields in excess of 90% for a prototypical heterojunction. Field, temperature, and band offset dependencies are investigated and found to be in agreement with earlier experiments. Whereas the investigated heterojunctions have substantial energy losses associated with the dissociation process, these results suggest that it is possible to reach high dissociation yields at low energy loss.     

P3HT/PCBM Bulk Heterojunction Solar Cells: Impact of Blend Composition and 3D Morphology on Device Performance

The performance of polymer solar cells (PSC) strongly depends on the 3D morphological organization of the donor and acceptor compounds within the bulk heterojunction active layer. The technique of electron tomography is a powerful tool for studying 3D morphology of the layers composed of poly(3‐hexylthiophene) (P3HT) and a fullerene derivative ([6,6]‐phenyl‐C61‐butyric acid methyl ester; PCBM), especially to quantify the amount and distribution of fibrillar P3HT nanocrystals throughout the volume of the active layer. In this study, electron tomography is used to characterize P3HT/PCBM layers with different blend compositions, both before and after thermal annealing. The power conversion efficiency of the corresponding PSCs is strongly dependent on the overall crystallinity of P3HT and the way P3HT crystals are distributed throughout the thickness of the active layer.     

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.     

Enhanced Performance in Polymer Solar Cells by Surface Energy Control

Enhanced performance of an inverted‐type polymer solar cell is reported by controlling the surface energy of a zinc oxide (ZnO) buffer layer, on which a photoactive layer composed of a polymer:fullerene‐derivative bulk heterojunction is formed. With the approach based on a mixed self‐assembled monolayer, the surface energy of the ZnO buffer layer can be controlled between 40 mN m^?1 and 70 mN m^?1 with negligible changes in its work function. For the given range of surface energy the power conversion efficiency increases from 3.27% to 3.70% through enhanced photocurrents. The optimized morphology obtained by surface energy control results in the enhanced photocurrent and transmission electron microscopy analysis verifies the correlation between the surface energy and the phase morphology of the bulk heterojunction. These results demonstrate that surface energy control is an effective method for further improving the performance of polymer solar cells, with potentially important implications for other organic devices containing an interface between a blended organic active layer and a buffer or an electrode layer.     

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.     

Nanostructured Si/Organic Heterojunction Solar Cells with High Open‐Circuit Voltage via Improving Junction Quality

Nanostructured silicon (Si) can provide improved light harvest efficiencies in organic‐Si heterojunction solar cells due to its low light reflection ratio compared with planar one. However, the associated large surface/volume ratio of nanostructured Si suffers from serious surface recombination as well as poor adhesion with organics in organic‐Si heterojunction solar cells, which leads to an inferior open‐circuit voltage (V_oc). Here, we develop a simple and effective method to suppress charge recombination as well as enhancing adhesion force between nanostructured Si and organics by incorporating a silane chemical, namely 3‐glycidoxypropyltrimethoxydsilane (GOPS). GOPS can chemically graft onto nanostructured Si and improve the aqueous organic wetting properties, suppressing surface charge recombination velocity dramatically. In addition, this chemically grafted layer can enhance adhesion force between organics and Si. In such a way, a record V_oc of 640 mV associated with a power conversion efficiency of 14.1% is obtained for organic‐nanostructured Si heterojunction devices. These findings suggest a promising approach to low‐cost and simple fabrication for high‐performance organic‐Si solar cells.     

Optimization of Si NC/P3HT Hybrid Solar Cells

Silicon nanocrystals (Si NCs) are shown to be an electron acceptor in hybrid solar cells combining Si NCs with poly(3‐hexylthiophene) (P3HT). The effects of annealing and different metal electrodes on Si NC/P3HT hybrid solar cells are studied in this paper. After annealing at 150 °C, Si NC/P3HT solar cells exhibit power conversion efficiencies as high as 1.47%. The hole mobility in the P3HT phase extracted from space‐charge‐limited current measurements of hole‐only devices increases from 2.48 × 10^?10 to 1.11 × 10^?9 m² V^?1 s^?1 after annealing, resulting in better transport in the solar cells. A quenching of the open‐circuit voltage and short‐circuit current is observed when high work function metals are deposited as the cathode on Si NC/P3HT hybrid devices.     

Real‐Time Investigation of Crystallization and Phase‐Segregation Dynamics in P3HT:PCBM Solar Cells During Thermal Annealing

Crystallization and phase segregation during thermal annealing lead to the increase of power‐conversion efficiency in poly(3‐hexylthiophene) (P3HT):[6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) bulk‐heterojunction solar cells. An understanding of the length and time scale on which crystallization and phase segregation occur is important to improve control of the nanomorphology. Crystallization is monitored by means of grazing incidence X‐ray diffraction in real time during thermal annealing. Furthermore, the change in film density is monitored by means of ellipsometry and the evolution of carrier mobilities by means of field effect transistors, both during annealing. From the combination of such measurements with those of device performance as a function of annealing time, it is concluded that the evolution of microstructure involves two important time windows: i) A first one of about 5 minutes duration wherein crystallization of the polymer correlates with a major increase of photocurrent; ii) a second window of about 30 minutes during which the aggregation of PCBM continues, accompanied by an increase in the fill factor.     

P3HT为空穴传输层的碳基钙钛矿太阳电池

碳电极具有成本低、印刷方便、可有效隔离水氧等优点,因此有望利用碳电极材料实现低成本、高稳定性的钙钛矿太阳电池。无空穴传输层的传统碳基钙钛矿太阳电池面临着空穴提取率低、电子逆向传输,钙钛矿和碳电极界面的载流子复合等问题。文章引入聚(3-己基噻吩)(P3HT)作为器件的空穴传输层,使碳基钙钛矿太阳电池ITO/SnO₂/MAPbI₃/P3HT/Carbon的光伏性能得到了显著改善:器件的光电转化效率从11.16% 提高到13.37%。在氮气环境下,连续光照1000h,太阳电池的光电转化效率可保持初始值的87%,而传统器件在光照500h后,其光电转化效率已下降至初始值的60%。     

Synthesis and Structure–Property Correlations of Dicyanovinyl‐Substituted Oligoselenophenes and their Application in Organic Solar Cells

The convergent synthesis of a series of acceptor–donor–acceptor (A‐D‐A) type dicaynovinyl (DCV)‐substituted oligoselenophenes DCVnS (n = 3–5) is presented. Trends in thermal and optoelectronic properties are studied, in dependence on the length of the conjugated backbone. Optical measurements reveal red‐shifted absorption spectra and electrochemical investigations show lowering of the lowest unoccupied molecular orbital (LUMO) energy levels for DCVnS compared to the corresponding thiophene analogs DCVnT. As a consequence, a lowering of the bandgap is observed. Single crystal X‐ray structure analysis of tetramer DCV4S provides important insight into the packing features and intermolecular interactions of the molecules, further corroborating the importance of the DCV acceptor groups for the molecular ordering. DCV4S and DCV5S are used as donor materials in planar heterojunction (PHJ) and bulk‐heterojunction (BHJ) organic solar cells. The devices show very high fill factors (FF), a high open circuit voltage, and power conversion efficiencies (PCE) of up to 3.4% in PHJ solar cells and slightly reduced PCEs of up to 2.6% in BHJ solar cells. In PHJ devices, the PCE for DCV4S almost doubles compared to the PCE reported for the oligothiophene analog DCV4T, while DCV5S shows an about 30% higher PCE than DCV5T.     

Star-Shaped Organic Semiconductor with Extraordinary Thermomechanical Property and Solution Processability for Stable Perovskite Solar Cells

Achieving the desired thermomechanical properties for highly solution-processable organic semiconductors is challenging but crucial for heat tolerance of emerging optoelectronic devices. To this end, the successful synthesis of triphenylene–ethylenedioxythiophene-dimethoxytriphenylamine (TP–ETPA), a star-shaped organic semiconductor, is reported through a direct arylation reaction that involves ETPA, an electron donor, being grafted densely onto TP, which possesses six electron-equivalent functionalization sites. Remarkably, TP–ETPA exhibits significantly improved hole mobility compared to 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD) at a given hole density, owing to its lower energetic disorder, larger average centroid distance, and smaller reorganization energy. TP–ETPA, with a molecular weight of 2888 Da and lacking flexible chains, demonstrates extraordinary solubility in nonpolar solvents, enabling the formation of dense, pinhole-free films through solution codeposition with an air-doping promoter. By utilizing the p-doped TP–ETPA composite as the hole transport layer, perovskite solar cells with an average power conversion efficiency of 23.4% are successfully fabricated. Notably, these devices display significantly enhanced operational stability and thermal stability at 85 °C. Molecular dynamics simulations reveal that the TP–ETPA-based hole transport layer possesses a high cohesive energy density, resulting in a large elastic modulus and slow diffusion of external species.