～1.2 V open-circuit voltage from organic solar cells

Organic solar cells (OSCs) have achieved rapid advance due to the continuous development of high-performance key materials.Recently,the power conversion efficiencies (PCEs)of OSCs under 1 Sun condition (AM 1.5 G,100 mW/cm2) are striving toward 19％[1-5].The PCE improvement benefits from the largely enhanced short-circuit current density (Jsc) and fill factor (FF).However,these cells show relatively low open-circuit voltage (Voc) around 0.8-0.9 V.The rise of Internet of Things (loT) industry has promoted the indoor application of solar cells.OSCs can afford higher PCEs under various indoor light as compared to 1 Sun condition[6,7],but they present lower Voc[8].Fabricating tandem devices is an effective strategy to boost the performance of OSCs.Sub-cells with syn-chronously high Voc,Jsc and FF are highly desired in tandem cells,while these sub-cells are still limited[9].Thus,improving Voc without sacrificing Jsc and FF is an urgent mission in OSCs.     

Blade-coated organic solar cells from non-halogenated solvent offer 17%efficiency

Organic solar cell(OSC)has attracted great interests due to its potential applications[1-9].To date,18%power conversion efficiency(PCE)has been achieved in single-junction OSC[10?13],indicating the feasibility of commercialization.This photovoltaic technology currently faces the performance gap between laboratory cells and large-area modules.     

Inorganic perovskite/organic tandem solar cells with efficiency over 20%

The rapid development of perovskite solar cells is beyond our imagination.The power conversion efficiency(PCE)of organic-inorganic hybrid perovskite solar cells has reached 25.5%(https://www.nrel.gov/pv/cell-efficiency.html).However,the unsatisfactory stability of hybrid perovskites is an obstacle for their commercialization,which results from the volatile and hygroscopic organic cations[1].     

Triarylamine-based dual-function coadsorbents with extended π-conjugation aryl linkers for organic dye-sensitized solar cells

Triarylamine-based dual-function coadsorbents containing a carboxylic acid acceptor linked by extended π-conjugation aryl linkers (e.g., phenylene: HC-A3, naphthalene: HC-A4 and anthracene: HC-A5) were newly designed and synthesized. They were used as coadsorbents in organic dye-sensitized solar cells (DSSCs) based on a porphyrin dye (hexyloxy-biphenyl-ZnP-CN-COOH (HOP)). For comparison, the π-conjugated phenyl linker (HC-A3) previously developed by our group was also used as a coadsorbent. The structural effects on the photophysical and electrochemical properties and DSSC performance were systematically investigated. As a result, the DSSCs based on HC-A4 and HC-5 displayed power conversion efficiencies (PCEs) of 8.2% and 5.1%, respectively, while the HC-A3-based DSSC achieved a PCE of 7.7%. In the case of HC-A4, both the short-circuit photocurrent densities (J_sc) and open-circuit voltages (V_oc) of DSSCs were simultaneously improved to a large extent due to the more effective prevention of π−π stacking of organic dye molecules and the better light-harvesting effect at short wavelengths. The HC-A5-based DSSC exhibited a much lower short-circuit current (J_sc) and open-circuit voltages (V_oc) compared to the HC-A4-based DSSC, due to the fact that the dihedral angle of the π-conjugated linkers was too high for electron injection into the TiO₂ conduction band (CB) level. This had a reduced effect on preventing the π−π stacking of dye molecules, resulting in lower J_sc and V_oc values.     

A–D–A type organic donors employing coplanar heterocyclic cores for efficient small molecule organic solar cells

Two linear organic A–D–A molecules (DTPT and DTPTT) comprised of electron-donating (D) coplanar heteroacenes as core end-capping with electron-accepting (A) dicyanovinylene were investigated as electron donor materials in organic photovoltaic (OPV) applications. The photophysical and electrochemical properties of these two dyes were examined. The A–D–A configuration renders these two molecules to have intense and red-shifted absorption characteristics for better light-harvesting (higher photocurrent density), while retaining relatively low HOMO energy levels for keeping sufficiently high open circuit voltage (V_oc) in OPV. The optical constants and molecular orientation of thin films were acquired with variable-angle spectroscopic ellipsometry (VASE). Due to the anisotropic behavior observed in thin film, these two organic donors were firstly adopted to combine with electron acceptor C₆₀ in a vacuum-processed planar heterojunction (PHJ) solar cells. The optimized DTPT-based PHJ device yielded a PCE of 3.01%, whereas the PHJ device based on DTPTT, delivered an inferior PCE of 1.70%. The exciton diffusion length extracted from spectrum-response modeling of PHJ devices is ∼5 nm and ∼4 nm for DTPT and DTPTT, respectively. Replacement of C₆₀ with C₇₀ for a better spectral response in 400–500 nm, planar-mixed heterojunction (PMHJ) SMOSCs without a thin donor layer in between the active layer and MoO₃ was found to produce optimum device results. The optimized DTPTT-based device showed a PCE of 3.02%, while the shorter counterpart DTPT delivered a PCE up to 5.64%.     

Modeling the structure–property relations in pillar-structured organic donor/acceptor solar cells

Recently, organic solar cells with ordered morphologies in the form of vertical, interpenetrating donor- and acceptor-pillars have been demonstrated with various fabrication techniques. In order to find the optimal shape and size of these pillar structures, the conventional computational method requires simulating and comparing across different pillar designs; this may be time consuming since the pillar designs could have a large number of variations. In this paper, we establish a theoretical and computational framework that allows for efficient optimization of pillar-type morphologies. We first capture the effects of two key morphological parameters – the specific donor/acceptor interfacial area and the donor/acceptor volume ratio – with closed-form structure–property relations. We then illustrate through three-dimensional device modeling that the photovoltaic behavior of these pillar-structured cells is essentially determined by these two morphological parameters. The cross-sectional pattern of the pillar structures, on the other hand, has no major influence on the cell performance. Finally, we demonstrate a fast procedure to generate a power-density map that can aid in designing the optimal pillar structures.     

Linear and propeller-like fluoro-isoindigo based donor–acceptor small molecules for organic solar cells

Two donor–acceptor type fluoro-isoindigo based small molecule semiconductors are synthesized and their optical, electrochemical, thermal, and charge transport properties are investigated. The two molecular chromophores differ by their architecture, linear (M1) vs propeller-like (M2). Both molecules present a broad absorption in the visible range and a low optical HOMO–LUMO gap (∼1.6 eV). AFM images of solution-processed thin films show that the trigonal molecule M2 forms highly oriented fibrils after a few seconds of solvent vapor annealing. The materials are evaluated as electron donor components in bulk heterojunction organic solar cells using PC₆₁BM as the electron acceptor. The devices based on the propeller-like molecule M2 exhibit a high open-circuit voltage (around 1.0 V) and a power conversion efficiency of 2.23%.     

Monolithic perovskite/silicon tandem solar cells offer an efficiency over 29％

Organic-inorganic hybrid perovskite semiconductors pos-sessing superior optoelectronic properties (e.g.long carrier dif-fusion lengths,high optical absorption coefficient,low ex-citon binding energy,and high defect tolerance) are attract-ing serious attention.The certified power conversion effi-ciency (PCE) for single-junction perovskite solar cells have ex-ceeded 25％[1,2].As a very promising PCE-enhancement strategy,tandem structure made by stacking a perovskite cell on a market-dominant silicon cell can yield much higher PCEs beyond the Shockley-Queisser limit of single-junction devices without adding substantial cost[3].To satisfy current-match-ing in tandem configuration,the top perovskite cell requires an ideal bandgap of ～1.7 eV rather than the ones (～1.5-1.6 eV)typically used for highly efficient single-junction perovskite devices given that the bottom silicon cell holds a bandgap of 1.12 eV[4].Such wide-bandgap perovskites achieved through I/Br alloying usually suffer from photoinduced phase segrega-tion and relatively low radiative efficiency,which inevitably result in large open-circuit voltage (Voc) deficits[5,6].Several strategies like adjusting perovskite composition[7,8],additive engineering[9,10],and upper surface passivation[11,12] have been utilized to stabilize these wide-bandgap perovskites and improve film quality to reduce Voc losses.The reported pe-rovskite/silicon tandem devices suffer from low Voc (＜1.9 V)and PCEs (≤28％)[13].There is still a large room for enhancing PCEs given that the predicted PCE limit is beyond 30％ for this tandem technology[14].     

MoO3–Au composite interfacial layer for high efficiency and air-stable organic solar cells

Efficient and stable polymer bulk-heterojunction solar cells based on regioregular poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PC₆₁BM) blend active layer have been fabricated with a MoO₃–Au co-evaporation composite film as the anode interfacial layer (AIL). The optical and electrical properties of the composite MoO₃–Au film can be tuned by altering the concentration of Au. A composite film with 30% (weight ratio) Au was used as the AIL and showed a better performance than both pure MoO₃ and PEDOT:PSS as AIL. The surface morphology of the MoO₃–Au composite film was investigated by atomic force microscopy (AFM) and showed that the originally rough ITO substrate became smooth after depositing the composite film, with the root mean square roughness (RMS) decreased from 4.08 nm to 1.81 nm. The smooth surface reduced the bias-dependent carrier recombination, resulting in a large shunt resistance and thus improving the fill factor and efficiency of the devices. Additionally, the air stability of devices with different AILs (MoO₃–Au composite, MoO₃ and PEDOT:PSS) were studied and it was found that the MoO₃–Au composite layer remarkably improved the stability of the solar cells with shelf life-time enhanced by more than 3 and 40 times compared with pure MoO₃ layer and PEDOT:PSS layer, respectively. We argue that the stability improvement might be related with the defect states in MoO₃ component.     

Low-LUMO 56π-electron fullerene acceptors bearing electron-withdrawing cyano groups for small-molecule organic solar cells

56π-Electron fullerene derivatives with electron-withdrawing cyano groups were synthesized by the reaction of 58π-electron fullerenes with NaCN, followed by in situ treatment with TsCN or MeOTf. The fullerene derivatives have low-lying LUMO levels, which are comparable with or lower than that of pristine C₆₀. They were used as solution-processable electron acceptors in small-molecule organic solar cells that showed a maximum power conversion efficiency of 2.0% (J_SC = 7.05 mA/cm², V_OC = 0.62 V and FF = 0.45) using chloroindium phthalocyanine as an electron donor.     

＞35％ 5-junction space solar cells based on the direct bonding technique

Multijunction solar cells are the highest efficiency photovoltaic devices yet demonstrated for both space and terrestri-al applications.In recent years five-junction cells based on the direct semiconductor bonding technique (SBT),demonstrates space efficiencies ＞35％ and presents application potentials.In this paper,the major challenges for fabricating SBT 5J cells and their appropriate strategies involving structure tunning,band engineering and material tailoring are stated,and 4-cm235.4％(AM0,one sun) 5J SBT cells are presented.Further efforts on detailed optical managements are required to improve the cur-rent generating and matching in subcells,to achieve efficiencies 36％-37％,or above.     

Electrical and photoelectrical properties of P3HT/n-Si hybrid organic–inorganic heterojunction solar cells

A detail analysis of electrical and photoelectrical properties of hybrid organic–inorganic heterojunction solar cells poly(3-hexylthiophene) (P3HT)/n-Si, fabricated by spin-coating of the polymeric thin film onto oxide passivated Si(1 0 0) surface, was carried out within the temperature ranging from 283 to 333 K. The dominating current transport mechanisms were established to be the multistep tunnel-recombination and space charge limited current at forward bias and leakage current through the shunt resistance at reverse bias. A simple approach was developed and successfully applied for the correct analysis of the high frequency C–V characteristics of hybrid heterojunction solar cells. The P3HT/n-Si solar cell under investigation possessed the following photoelectric parameters: J_sc = 16.25 mA/cm², V_oc = 0.456 V, FF = 0.45, η = 3.32% at 100 mW/cm² AM 1.5 illumination. The light dependence of the current transport mechanisms through the P3HT/n-Si hybrid solar cells is presented quantitatively and discussed in detail.     

Oligothiophene-based small molecules with 3,3′-difluoro-2,2′-bithiophene central unit for solution-processed organic solar cells

Two new oligothiophene-based small molecules, namely DRCN6T-F and DRCN8T-F, with 3,3′-difluoro-2,2′-bithiophene as the central building block and 2-(1,1-dicyanomethylene)-rhodanine as end groups, were designed and synthesized. Compared to their non-fluorinated counterparts DRCN6T and DRCN8T, DRCN6T-F and DRCN8T-F exhibit enhanced intermolecular interactions and lower HOMO energy levels. However, PCEs of 2.26% and 5.07% were obtained for DRCN6T-F and DRCN8T-F based optimized devices, respectively, lower than those of non-fluorinated molecules DRCN6T and DRCN8T. The relatively poor performance for the DRCN6T-F and DRCN8T-F were mainly caused by their low short-circuit current densities, due to their unfavorable morphologies and low charge carrier mobilities.     

CdSe-sensitized inorganic–organic heterojunction solar cells: The effect of molecular dipole interface modification and surface passivation

CdSe-sensitized heterojunction solar cells composed of mesoscopic TiO₂/CdSe/P3HT (poly-3-hexylthiophene) were constructed, and the negative molecular dipole of 4-methoxybenzenethiol (MBT) and the ZnS passivation layer were used as interface modifiers to improve device performance. Through the interface modification between TiO₂/CdSe and P3HT using MBT and by ZnS surface passivation, the power conversion efficiency of the modified solar cell was greatly enhanced from 1.02% to 1.62% under 1 sun illumination.     

MoO3/Au/MoO3–PEDOT:PSS multilayer electrodes for ITO-free organic solar cells

The effect of the MoO₃–PEDOT:PSS composite layer in the MoO₃/Au/MoO₃–PEDOT:PSS multilayer electrode on the power conversion efficiency of ITO-free organic solar cells (OSCs) was evaluated. The MoO₃ (30 nm)/Au(12 nm)/MoO₃–PEDOT:PSS (30 nm)/PEDOT:PSS structure showed ~7% more optical transmittance than the MoO₃ (30 nm)/Au (12 nm)/MoO₃(30 nm)/PEDOT:PSS structure at 550 nm wavelength. The OSCs using MoO₃/Au/MoO₃–PEDOT:PSS multilayer electrodes as anodes showed a considerable improvement in power conversion efficiency (PCE), from 1.84% to 2.81%, comparable to ITO based OSCs with PCE of 2.89%. This improvement is attributed to the suppression of MoO₃ dissolution by the acidic hole transport layer (HTL) PEDOT:PSS on the MoO₃/Au/MoO₃–PEDOT:PSS multilayer electrode, resulting in high J_sc, V_oc and FF of the OSCs. This composite based multilayer electrode was shown to be a promising replacement in ITO-free flexible optoelectronic devices.     

Interfacial layer material derived from dialkylviologen and sol–gel chemistry for polymer solar cells

Sol–gel processible organosilicate material based on dialkylviologen (1,1-(bis-trimethoxysilane)-[4,4′]bipyridium dibromide (bis-trimethoxypropylsilane)-yl-viologen, PV-Si) was synthesized and used as an interfacial layer material for polymer solar cells based on poly(3-hexylthiophene): [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM). PV-Si is very good soluble in polar protic solvents because of two pyrinium bromide salts and PV-Si pre-polymer can be easily prepared by sol–gel chemistry under the mild acidic conditions. From the ultraviolet spectroscopy (UPS) study, the reduction of the work function of Al and ITO is observed by the formation of interface dipole, which is induced by the thin film of thermally cured PV-Si pre-polymer (cPV-Si) at 180 °C. The open circuit voltage (V_oc) of conventional type polymer solar cell (CPSC) with a structure of ITO/active layer (P3HT:PCBM)/cPV-Si(<5 nm)/Al is 0.58 V, which is higher than the CPSC without cPV-Si (0.55 V). This indicates that the favorable interface dipole is generated by the thin film of cPV-Si. Besides, the power conversion efficiency (PCE) of CPSC with cPV-Si reaches at 2.90%, which is higher than that of the device without cPV-Si (2.69%). Surprisingly, the PCE and the short circuit current (J_sc) of inverted type polymer solar cell (IPSC) with a structure of ITO/cPV-Si (<5 nm)/active layer/WO₃/Ag are 2.83% and ?9.19 mA/cm², respectively, which are higher than those of the device with ZnO (2.51% and ?8.63 mA/cm²) as an electron transporting/injecting layer. This is due to that the work function of ITO is also reduced by the formation of interface dipole. The IPSC with cPV-Si as an interfacial layer (IFL) shows very good rectification and a contact property as well. From the results, the thin layer of cPV-Si is potential material for an IFL for either CPSC or IPSC. Especially, ZnO can be replaced by cPV-Si because of their improved device performances and pretty low processing temperature.     

Acetylene-bridged D–A–D type small molecule comprising pyrene and diketopyrrolopyrrole for high efficiency organic solar cells

To explore effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPP-A-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were successfully synthesized and characterized. (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on a blend of HD-DPP-A-PY as a donor and [6,6]-phenyl-C₇₁-butyric-acid-methyl-ester (PC₇₀BM) as an acceptor exhibited PCEs as high as 3.15%.     

High open-circuit voltage of the solution-processed organic solar cells based on benzothiadiazole–triphenylamine small molecules incorporating π-linkage

Two novel small molecular photovoltaic (PV) materials, BDPTBT and BDATBT were designed and synthesized, consisting of 5,6-bis-(octyloxy)benzo[c][1,2,5]thiadiazole (DOBT) as electron-withdrawing core (A), and triphenylamine (TPA) as electron-donating side group (D). Moreover, the benzene and ethynylbenzene as π-linkage were introduced to form donor–π-acceptor–π-donor (D–π-A–π-D) typed molecular structures, respectively. To fully investigate the linkage effect of a series of small molecules, two reference compounds BDCTBT and BDETBT were also studied systematically, consisting of 2-phenylacrylonitrile and styrene as π-linkage, respectively. As a result, the π-linkage units, benzene, styrene, ethynylbenzene and 2-phenylacrylonitrile played an important role in modifying molecular structure and improving PV performance. Bulk heterojunction (BHJ) solar cells based on BDPTBT/PC₆₁BM and BDATBT/PC₆₁BM yielded the power conversion efficiencies (PCEs) of 2.99% and 2.03%, respectively. Notably, BDATBT based device showed a high open-circuit voltage (Voc) of 1.03 V. Compared to the results we have reported previously, the reference devices based on BDCTBT/PC₆₁BM and BDETBT/PC₆₁BM with the optimized weight ratio showed dramatically enhanced PCEs of 4.84% and 3.40%, respectively, and BDCTBT based device showed a high Voc of 1.08 V. To our knowledge, the Voc of 1.08 V is the highest voltage reported to date for devices prepared from solution-processed small-molecule-donor materials, and the PCE of 4.84% is the highest efficiency reported so far for D–A–D-typed benzothiadiazole (BT)–TPA based solution-processed small molecules PV devices.     

D–π–A organic dyes with various bulky amine-typed donor moieties for dye-sensitized solar cells employing the cobalt electrolyte

SGT dyes containing various amine-typed donors as triphenylamine, bis-fluorenylamine and bis-phenothiazinylamine as the electron donor and a cyanoacrylic acid moiety as electron acceptor in D–π–A system, were developed to use in dye-sensitized solar cells (DSSCs). The SGT-102 dye containing bis-fluorenylamine had a better prevented charge recombination than other SGT dyes; leading to improvement in V_oc. As a result, the conversion efficiency of 7.22% was achieved with a J_sc of 12.1 mA cm⁻², V_oc of 865 mV and a FF of 69.1 for the DSSC employing a dye containing the bulky bis-fluorenylamine donor unit, while the DSSC based on a dye containing the bulky bis-phenothiazinylamine donor unit showed a lower J_sc and V_oc, leading to a lower efficiency of 5.16%, due to slow charge recombination associated with differently geometric structure orientations.     

Benzo[1,2-b:4,5-b′]dithiophene and benzotriazole based small molecule for solution-processed organic solar cells

A novel deep HOMO A₁-π-A₂-D-A₂-π-A₁ type molecule (D(CATBTzT)BDT), which terminal electron-withdrawing octyl cyanoacetate group is connected to a benzo[1,2-b:4,5-b′]dithiophene (BDT) core through another electron-accepting benzotriazole block, has been synthesized, characterized, and employed as electron donor material for small molecule organic solar cells (SM-OSCs). By simple solution spin-coating fabrication process, D(CATBTzT)BDT/PC₆₁BM based OSCs exhibit a power conversion efficiency (PCE) of 3.61% with a high open-circuit voltage of 0.93 V. The D(CATBTzT)BDT based solar cells device also can show high FF of 72% with PCEs of 2.31% which is one of the best FF results for solution-processed SM-OSCs.