Alternating Copolymers of Cyclopenta[2,1‐b;3,4‐b′]dithiophene and Thieno[3,4‐c]pyrrole‐4,6‐dione for High‐Performance Polymer Solar Cells

A series of alternating copolymers of cyclopenta[2,1‐b;3,4‐b′]dithiophene (CPDT) and thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) have been prepared and characterized for polymer solar cell (PSC) applications. Different alkyl side chains, including butyl (Bu), hexyl (He), octyl (Oc), and 2‐ethylhexyl (EH), are introduced to the TPD unit in order to adjust the packing of the polymer chain in the solid state, while the hexyl side chain on the CPDT unit remains unchanged to simplify discussion. The polymers in this series have a simple main chain structure and can be synthesized easily, have a narrow band gap and a broad light absorption. The different alkyl chains on the TPD unit not only significantly influence the solubility and chain packing, but also fine tune the energy levels of the polymers. The polymers with Oc or EH group have lower HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels, resulting higher open circuit voltages (V_oc) of the PSC devices. Power conversion efficiencies (PCEs) up to 5.5% and 6.4% are obtained from the devices of the Oc substituted polymer (PCPDTTPD‐Oc) with PC₆₁BM and PC₇₁BM, respectively. This side chain effect on the PSC performance is related to the formation of a fine bulk heterojunction structure of polymer and PCBM domains, as observed with atomic force microscopy.     

Wide Bandgap Copolymers Based on Quinoxalino[6,5‐f].quinoxaline for Highly Efficient Nonfullerene Polymer Solar Cells

Two novel wide bandgap copolymers based on quinoxalino[6,5‐f]quinoxaline (NQx) acceptor block, PBDT–NQx and PBDTS–NQx, are successfully synthesized for efficient nonfullerene polymer solar cells (PSCs). The attached conjugated side chains on both benzodithiophene (BDT) and NQx endow the resulting copolymers with low‐lying highest occupied molecular orbital (HOMO) levels. The sulfur atom insertion further reduces the HOMO level of PBDTS–NQx to ?5.31 eV, contributing to a high open‐circuit voltage, V _oc, of 0.91 V. Conjugated n ‐octylthienyl side chains attached on the NQx skeletons also significantly improve the π–π* transitions and optical absorptions of the copolymers in the region of short wavelengths, which induce a good complementary absorption when blending with the low bandgap small molecular acceptor of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene. The wide absorption range makes the active blends absorb more photons, giving rise to a high short‐circuit current density, J _sc, value of 15.62 mA cm^?2. The sulfur atom insertion also enhances the crystallinity of PBDTS–NQx and presents its blend film with a favorable nanophase separation, resulting in improved J _sc and fill factor (FF) values with a high power conversion efficiency of 11.47%. This work not only provides a new fused ring acceptor block (NQx) for constructing high‐performance wide bandgap copolymers but also provides the NQx‐based copolymers for achieving highly efficient nonfullerene PSCs.     

Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open‐Circuit Voltage

In organic solar cells based on polymer:fullerene blends, energy is lost due to electron transfer from polymer to fullerene. Minimizing the difference between the energy of the polymer exciton (E_D*) and the energy of the charge transfer state (E_CT) will optimize the open‐circuit voltage (V_oc). In this work, this energy loss E_D*‐E_CT is measured directly via Fourier‐transform photocurrent spectroscopy and electroluminescence measurements. Polymer:fullerene photovoltaic devices comprising two different isoindigo containing polymers: P3TI and PTI‐1, are studied. Even though the chemical structures and the optical gaps of P3TI and PTI‐1 are similar (1.4 eV–1.5 eV), the optimized photovoltaic devices show large differences in V_oc and internal quantum efficiency (IQE). For P3TI:PC₇₁BM blends a E_D*‐E_CT of ～ 0.1 eV, a V_oc of 0.7 V and an IQE of 87% are found. For PTI‐1:PC₆₁BM blends an absence of sub‐gap charge transfer absorption and emission bands is found, indicating almost no energy loss in the electron transfer step. Hence a higher V_oc of 0.92 V, but low IQE of 45% is obtained. Morphological studies and field dependent photoluminescence quenching indicate that the lower IQE for the PTI‐1 system is not due to a too coarse morphology, but is related to interfacial energetics. Losses between E_CT and qV_oc due to radiative and non‐radiative recombination are quantified for both material systems, indicating that for the PTI‐1:PC₆₁BM material system, V_oc can only be increased by decreasing the non‐radiative recombination pathways. This work demonstrates the possibility of obtaining modestly high IQE values for material systems with a small energy offset (<0.1 eV) and a high V_oc.     

Simulation and fabrication of heterojunction silicon solar cells from numerical computer and hot‐wire CVD

In this paper, we will present a Pc1D numerical simulation for heterojunction (HJ) silicon solar cells, and discuss their possibilities and limitations. By means of modeling and numerical computer simulation, the influence of emitter‐layer/intrinsic‐layer/crystalline‐Si heterostructures with different thickness and crystallinity on the solar cell performance is investigated and compared with hot wire chemical vapor deposition (HWCVD) experimental results. A new technique for characterization of n‐type microcrystalline silicon (n‐µc‐Si)/intrinsic amorphous silicon (i‐a‐Si)/crystalline silicon (c‐Si) heterojunction solar cells from Pc1D is developed. Results of numerical modeling as well as experimental data obtained using HWCVD on µc‐Si (n)/a‐Si (i)/c‐Si (p) heterojunction are presented. This work improves the understanding of HJ solar cells to derive arguments for design optimization. Some simulated parameters of solar cells were obtained: the best results for J_sc = 39·4 mA/cm², V_oc = 0·64 V, FF = 83%, and η = 21% have been achieved. After optimizing the deposition parameters of the n‐layer and the H₂ pretreatment of solar cell, the single‐side HJ solar cells with J_sc = 34·6 mA/cm², V_oc = 0·615 V, FF = 71%, and an efficiency of 15·2% have been achieved. The double‐side HJ solar cell with J_sc = 34·8 mA/cm², V_oc = 0·645 V, FF = 73%, and an efficiency of 16·4% has been fabricated. Copyright © 2009 John Wiley & Sons, Ltd.     

Subtle Polymer Donor and Molecular Acceptor Design Enable Efficient Polymer Solar Cells with a Very Small Energy Loss

A new wide bandgap polymer donor, PNDT‐ST, based on naphtho[2,3‐b:6,7‐b′]dithiophene (NDT) and 1,3‐bis(thiophen‐2‐yl)‐5,7‐bis(2‐ ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione (BDD) is developed for efficient nonfullerene polymer solar cells. To better match the energy levels, a new near infrared small molecule of Y6‐T is also developed. The extended π‐conjugation and less twist of PNDT‐ST provides it with higher crystallinity and stronger aggregation than the PBDT‐ST counterpart. The higher lowest occupied molecular orbital level of Y6‐T than Y6 favors the better energy level match with these polymers, resulting in improved open circuit voltage (V_oc) and power conversion efficiency (PCE). The high crystallinity and strong aggregation of PNDT‐ST also induces large phase separation with poorer morphology, leading to lower fill factor and reduced PCE than PBDT‐ST. To mediate the crystallinity and optimize the morphology, PNDT‐ST and PBDT‐ST are blended together with Y6‐T, forming the ternary blend devices. As expected, the two compatible polymers allow continual optimization of the morphology by varying the blend ratio. The optimized ternary blend devices deliver a champion PCE as high as 16.57% with a very small energy loss (E_loss) of 0.521 eV. Such small E_loss is the best record for polymer solar cells with PCEs over 16% to date.     

Structural Origins for Tunable Open‐Circuit Voltage in Ternary‐Blend Organic Solar Cells

Ternary‐blend bulk‐heterojunction solar cells have provided a unique opportunity for tuning the open‐circuit voltage (V_oc) as the “effective” highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels shift with active‐layer composition. Grazing‐incidence X‐ray diffraction (GIXD) measurements performed on such ternary‐blend thin films reveal evidence that the two polymer donors interact intimately; their ionization potentials are thus reflections of the blend compositions. In ternary‐blend thin films in which the two polymer donors do not interact physically, the polymer donors each retain their molecular electronic character; solar cells constructed with these ternary blends thus exhibit V_ocs that are pinned to the energy level difference between the highest of the two lying HOMO and the LUMO of the electron acceptor. These observations are consistent with the organic alloy model proposed earlier. Quantification of the square of the square‐root differences of the surface energies of the components provides a proxy for the Flory–Huggins interaction parameter for polymer donor pairs in these ternary‐blend systems. Of the three ternary‐blend systems examined herein, this quantity has to be below 0.094 in order for ternary‐blend solar cells to exhibit tunable V_oc.     

Achieving Organic Solar Cells with an Efficiency of 18.80% by Reducing Nonradiative Energy Loss and Tuning Active Layer Morphology

The efficiency of organic solar cells (OSCs) is primarily limited by their significant nonradiative energy loss and unfavorable active layer morphology. Achieving high-efficiency OSCs by suppressing nonradiative energy loss and tuning the active layer morphology remains a challenging task. In this study, an acceptor named CH-ThCl is designed, featuring an extended conjugation central core, dichlorodithienoquinoxaline. The incorporation of chlorine-substituted extended conjugation in the central core enhances the acceptor's rigidity and promotes J-aggregation, leading to improved molecular luminescent efficiency and a reduction in nonradiative energy loss. A binary device based on PM6: CH-ThCl demonstrates a power conversion efficiency (PCE) of 18.16% and exhibits a high open-circuit voltage (V_oc) of 0.934 V, attributed to the remarkably low nonradiative energy loss of 0.21 eV. Furthermore, a ternary device is fabricated by incorporating CH-6F as the third component, resulting in a significantly enhanced PCE of 18.80%. The ternary device exhibits improvements in short-circuit current (J_sc) and fill factor (FF) while maintaining the V_oc, primarily due to the optimized active layer morphology. These results highlight the effectiveness of combining the reduction of nonradiative energy loss and precise tuning of the active layer morphology as a viable strategy for achieving high-efficiency OSCs.     

Development of a‐SiOx:H solar cells with very high Voc × FF product

In this study, deposition conditions for making a‐SiO_x:H are investigated systematically in order to obtain a high band gap material. We found that at given optical band gap, a‐SiO_x:H with favorable opto‐electronic properties can be obtained when deposited using low CO₂ flow rates and deposition pressures. We also found that a low radio frequency power density is required in order to limit the effect of ion bombardment on the material properties of i‐a‐SiO_x:H and thereby the solar cell performance. In addition, by decreasing the heater temperature from 300 to 200°C when making the i‐a‐SiO_x:H, the V_oc can be increased. We employed optimized p‐doped and n‐doped a‐SiO_x:H films into the p‐i‐n solar cells, and as a consequence, a high V_oc of over 1 V and high fill factor (FF) are obtained. When depositing on texture‐etched ZnO:Al substrates, a high efficiency a‐SiO_x:H single junction solar cell having a high V_oc × FF product of 0.761 (V_oc: 1.042 V, J_sc: 10.3 mA/cm², FF: 0.73, efficiency: 7.83%) was obtained. The a‐SiO_x:H solar cell shows comparable light degradation characteristics to standard a‐Si:H solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

High‐Yield Synthesis and Electrochemical and Photovoltaic Properties of Indene‐C70 Bisadduct

[6, 6]‐Phenyl‐C₆₁‐butyric acid methyl ester (PC₆₀BM) is the widely used acceptor material in polymer solar cells (PSCs). Nevertheless, the low LUMO energy level and weak absorption in visible region are its two weak points. For enhancing the solar light harvest, the soluble C₇₀ derivative PC₇₀BM has been used as acceptor instead of PC₆₀BM in high efficiency PSCs in recent years. But, the LUMO level of PC₇₀BM is the same as that of PC₆₀BM, which is too low for the PSCs based on the polymer donors with higher HOMO level, such as poly (3‐hexylthiophene) (P3HT). Here, a new soluble C₇₀ derivative, indene‐C₇₀ bisadduct (IC₇₀BA), is synthesized with high yield of 58% by a one‐pot reaction of indene and C₇₀ at 180 °C for 72 h. The electrochemical properties and electronic energy levels of the fullerene derivatives are measured by cyclic voltammetry. The LUMO energy level of IC₇₀BA is 0.19 eV higher than that of PC₇₀BM. The PSC based on P3HT with IC₇₀BA as acceptor shows a higher V_oc of 0.84 V and higher power conversion efficiency (PCE) of 5.64%, while the PSC based on P3HT/PC₆₀BM and P3HT/PC₇₀BM displays V_oc of 0.59 V and 0.58 V, and PCE of 3.55% and 3.96%, respectively, under the illumination of AM1.5G, 100 mW cm^?2. The results indicate that IC₇₀BA is an excellent acceptor for the P3HT‐based PSCs and could be a promising new acceptor instead of PC₇₀BM for the high performance PSCs based on narrow bandgap conjugated polymer donor.     

Isoindigo-based polymer solar cells with high open circuit voltages up to 1.01 V

Isoindigo-based copolymers containing non-fluorinated (PIIDBT) and fluorinated bithiophene moieties (PIIDFBT) were synthesized. The replacement of hydrogen atoms with electronegative fluorine atom attained the intramolecular noncovalent interactions in conjugated polymer structure employing the concept of conformational locks. Furthermore, the fluorine characteristics as substituent atoms tuned the electron withdrawing ability and made the HOMO energy levels of fluorinated moieties deeper (−5.79 eV) than that of non-fluorinated polymer (−5.54 eV). As a result, the photovoltaic performance of PIIDFBT was improved by increasing its open circuit voltage (V_oc) up to 1.01 V, resulting in a power conversion efficiency of 6.21%. The PIIDFBT with high V_oc can be recognized as a promising candidate for both single junction and multi-junction photovoltaics.     

10.8% Efficiency Polymer Solar Cells Based on PTB7‐Th and PC71BM via Binary Solvent Additives Treatment

In this work, polymer solar cells are fabricated based on the blend of PTB7‐Th: PC₇₁BM by using a mixed solvent additive of 1,8‐diiodooctane and N‐methyl pyrrolidone to optimize the morphology of the blend. A high power conversion efficiency (PCE) of 10.8% has been achieved with a simple conventional device. In order to deeply investigate the influence of the mixed solvent additives on the morphology and device performance, the variations of the molecular packing and bulk morphology of the blend film cast from ortho‐dichlorobenzene with single or binary solvent additives are measured. Although all the blend films exhibit similar domain size and nanoscale phase separation, the blend film processed with mixed solvent additive shows the highest domain purity, resulting in the least bimolecular recombination, relatively high J_sc and FF, and hence enhanced PCE. Therefore, the best photovoltaic performance with the V_oc of 0.82 V, J_sc of 19.1 mA cm^?2, FF of 69.1%, and PCE of 10.8% are obtained for the device based on the blend with binary solvent additive treatment.     

Improving the open-circuit voltage of alkylthio-substituted photovoltaic polymers via post-oxidation

Molecular designing of photovoltaic polymers based on benzodithiophene (BDT) building blocks for high power conversion efficiency (PCE) in polymer solar cells (PSCs) arouse much attention in the past few years. To meliorate the energy levels of photovoltaic polymers featuring alkylthio substituted BDT units, a novel post-polymerization oxidation method was proposed applied in converting sulfur atom into sulfonyl group on side chains of the pristine polymer PBT-S. After treating with tiny amount of meta-chloroperoxybenzoic acid (m-CPBA) and hydrogen peroxide (H₂O₂), two batches of the target polymers, namely, PBT-SO₂-M and PBT-SO₂-H were prepared for the first time, respectively. The photochemical and electrochemical results indicate that both the HOMO levels are distinctly dropped with almost no influence on band gaps by introducing strong electron-withdrawing sulfonyl groups on side chains of BDT. Accordingly, the photovoltaic results reveal that the V_oc of devices based on PBT-SO₂-M and PBT-SO₂-H are 0.81, 0.71 V which are 0.17 and 0.07 V higher than that of pristine polymer PBT-S, respectively. Moreover, the J_sc and PCE of PBT-SO₂-H devices are comparable with those of the devices based on PBT-S. Overall, this work suggests that the molecular energy levels of D–A copolymers can be effectively tuned by a post-oxidation method.     

Two new A-D-A type small molecule acceptors based on C2v-symmetric dithienocyclopentaspiro[fluorene-9,9′-xanthene] core for polymer solar cells

A C_2v-symmetric core, dithienocyclopentaspiro[fluorene-9,9′-xanthene], was used as the central block for the first time to design and synthesize A-D-A type small molecule acceptors for nonfullerene polymer solar cells (PSCs), and two new small molecule acceptors of TSFX-2F and TSFX-4F were synthesized based on the C_2v-symmetric core. The two TSFX-based acceptors show high thermal stability, strong absorption in the wavelength region of 550–750 nm and appropriate energy levels. The PSCs with the broad bandgap polymer J71 as donor and TSFX-2F as acceptor demonstrated power conversion efficiency (PCE) of 9.42% with open circuit voltage (V_oc) of 0.89 V, short circuit current density (J_sc) of 15.27 mA cm⁻² and fill factor (FF) of 69.30%, while the PSC based on J71:TSFX-4F shows a PCE of 8.47% with V_oc of 0.83 V, J_sc of 15.48 mA cm⁻² and FF of 66.16%. The higher V_oc of the PSC based on J71: TSFX-2F is benefitted from the up-shifted LUMO energy level of the TSFX-2F acceptor, and its higher FF can be ascribed to the higher and more balanced hole and electron mobilities of the J71: TSFX-2F active layer. This work demonstrates that the new C_2v-symmetric building block is a promising central D-unit for the design and synthesis of new structured norfullerene acceptors for high-performance PSCs.     

Nonfullerene Polymer Solar Cells based on a Perylene Monoimide Acceptor with a High Open‐Circuit Voltage of 1.3 V

Nonfullerene polymer solar cells (PSCs) are fabricated with a perylene monoimide‐based n‐type wide‐bandgap organic semiconductor PMI‐F‐PMI as an acceptor and a bithienyl‐benzodithiophene‐based wide‐bandgap copolymer PTZ1 as a donor. The PSCs based on PTZ1:PMI‐F‐PMI (2:1, w/w) with the treatment of a mixed solvent additive of 0.5% N ‐methyl pyrrolidone and 0.5% diphenyl ether demonstrate a very high open‐circuit voltage (V _oc) of 1.3 V with a higher power conversion efficiency (PCE) of 6%. The high V _oc of the PSCs is a result of the high‐lying lowest unoccupied molecular orbital (LUMO) of ?3.42 eV of the PMI‐F‐PMI acceptor and the low‐lying highest occupied molecular orbital (HOMO) of ?5.31 eV of the polymer donor. Very interestingly, the exciton dissociation efficiency in the active layer is quite high, even though the LUMO and HOMO energy differences between the donor and acceptor materials are as small as ≈0.08 and 0.19 eV, respectively. The PCE of 6% is the highest for the PSCs with a V _oc as high as 1.3 V. The results indicate that the active layer based on PTZ1/PMI‐F‐PMI can be used as the front layer in tandem PSCs for achieving high V _oc over 2 V.     

17.87% Efficiency All-Polymer Tandem Solar Cell Enabled by Complementary Absorbing Polymer Acceptors

All-polymer solar cells (all-PSCs) possess distinguished advantages of excellent morphology stability, thermal stability, and mechanical flexibility. Tandem solar cells, by stacking two sub-cells, can absorb more photons in a wider wavelength range and can reduce thermal losses. However, limitation of polymer acceptors with suitable bandgaps hinders the development of tandem all-PSCs. Herein, highly efficient tandem all-PSCs are fabricated by employing two polymerized small molecular acceptors (PSMAs) of wide bandgap PIDT (1.66 eV) in the front cell and narrow bandgap PY-IT (1.4 eV) in the rear cell. The two sub-cells with the polymer donors of PM7 in front cell and PM6 in rear cell show high open circuit voltage (V_oc) of 1.10 V for the front cell and 0.94 V for the rear cell. By rational device optimizations, the best power conversion efficiency of 17.87% is achieved for the tandem all-PSCs with high V_oc of 2.00 V. 17.87% is one of the highest efficiency for the all-PSCs, and 2.00 V is one of the highest V_oc for all the tandem organic solar cells. Moreover, the tandem all-PSCs show excellent thermal and light-soaking stability compared with their small-molecule counterparts. The results provide insight to the potential of bandgap tuning in PSMAs, and indicate that the tandem architecture is an effective strategy to boost performance of the all-PSCs.     

Fluorobenzotriazole (FTAZ)‐Based Polymer Donor Enables Organic Solar Cells Exceeding 12% Efficiency

The fluorobenzotriazole (FTAZ)‐based copolymer donors are promising candidates for nonfullerene polymer solar cells (PSCs), but suffer from relatively low photovoltaic performance due to their unsuitable energy levels and unfavorable morphology. Herein, three polymer donors, L24 , L68 , and L810 , based on a chlorinated‐thienyl benzodithiophene (BDT‐2Cl) unit and FTAZ with different branched alkyl side chain, are synthesized. Incorporation of a chlorine (Cl) atom into the BDT unit is found to distinctly optimize the molecular planarity, energy levels, and improve the polymerization activity. Impressively, subtle side chain length of FTAZ realizes a dramatic improvement in all the device parameters, as revealed by the short‐current density (J_sc) improved from 7.41 to 20.76 mA cm^?2, fill‐factor from 36.3 to 73.5%, and even the open‐circuit voltage (V_oc) from 0.495 to 0.790 V. The best power conversion efficiency (PCE) of 12.1% is obtained from the L810‐based device, which is one of the highest values reported for FTAZ‐based PSCs so far. Notably, the corresponding external quantum efficiency curve keeps a very prominent value up to 80% from 500 to 800 nm. The notable performance is discovered from the reduced energy loss, improved molecular face‐on orientation, the down‐shifted energy levels, and optimized absorption coefficient regulated by side‐chain engineering.     

Origin of Open‐Circuit Voltage Enhancements in Planar Perovskite Solar Cells Induced by Addition of Bulky Organic Cations

The origin of performance enhancements in p‐i‐n perovskite solar cells (PSCs) when incorporating low concentrations of the bulky cation 1‐naphthylmethylamine (NMA) are discussed. A 0.25 vol % addition of NMA increases the open circuit voltage (V_oc) of methylammonium lead iodide (MAPbI₃) PSCs from 1.06 to 1.16 V and their power conversion efficiency (PCE) from 18.7% to 20.1%. X‐ray photoelectron spectroscopy and low energy ion scattering data show NMA is located at grain surfaces, not the bulk. Scanning electron microscopy shows combining NMA addition with solvent assisted annealing creates large grains that span the active layer. Steady state and transient photoluminescence data show NMA suppresses non‐radiative recombination resulting from charge trapping, consistent with passivation of grain surfaces. Increasing the NMA concentration reduces device short‐circuit current density and PCE, also suppressing photoluminescence quenching at charge transport layers. Both V_oc and PCE enhancements are observed when bulky cations (phenyl(ethyl/methyl)ammonium) are incorporated, but not smaller cations (Cs/MA)—indicating size is a key parameter. Finally, it demonstrates that NMA also enhances mixed iodide/bromide wide bandgap PSCs (V_oc of 1.22 V with a 1.68 eV bandgap). The results demonstrate a facile approach to maximizing V_oc and provide insights into morphological control and charge carrier dynamics induced by bulky cations in PSCs.     

Highly Efficient and Thermally Stable Polymer Solar Cells with Dihydronaphthyl‐Based [70]Fullerene Bisadduct Derivative as the Acceptor

The efficiency of polymer solar cells (PSCs) can be essentially enhanced by improving the performance of electron‐acceptor materials, including by increasing the lowest unoccupied molecular orbital (LUMO) level, improving the optical absorption, and tuning the material solubility. Here, a new soluble C₇₀ derivative, dihydronaphthyl‐based C₇₀ bisadduct (NC₇₀BA), is synthesized and explored as acceptor in PSCs. The NC₇₀BA has high LUMO energy level that is 0.2 eV higher than [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), and displays broad light absorption in the visible region. Consequently, the PSC based on the blend of poly(3‐hexylthiophene) (P3HT) and NC₇₀BA shows a high open‐circuit voltage (V_oc = 0.83 V) and a high power conversion efficiency (PCE = 5.95%), which are much better than those of the P3HT:PCBM‐based device (V_oc = 0.60 V; PCE = 3.74%). Moreover, the amorphous nature of NC₇₀BA effectively suppresses the thermally driven crystallization, leading to high thermal stability of the P3HT:NC₇₀BA‐based solar cell devices. It is observed that the P3HT:NC₇₀BA‐based device retains 80% of its original PCE value against thermal heating at 150 °C over 20 h. The results unambiguously indicate that the NC₇₀BA is a promising acceptor material for practical PSCs.     

Improving the open‐circuit voltage of Cu2ZnSnSe4 thin film solar cells via interface passivation

This study reports on substantial improvement of the open‐circuit voltage (V _oc) of Cu₂ZnSnSe₄ (CZTSe) thin film solar cells by applying a passivation strategy to both the top and bottom interfaces of the CZTSe absorber, which involves insertion of a thin dielectric layer between the CZTSe and the surrounding layers. The study also presents in‐depth material characterizations using transmission electron microscopy, energy dispersive X‐ray spectroscopy, low‐temperature photoluminescence, and secondary ion mass spectrometry, to reveal the effects of the interface passivation. To passivate the bottom Mo/CZTSe interface, a dielectric layer with patterned local contacts of dimensions down to ~100 nm was prepared using nanosphere lithography. With this, the V _oc, short‐circuit current, and fill factor (FF) were significantly enhanced due to reduction in carrier recombination at the bottom Mo/CZTSe interface. The top CZTSe/CdS interface was passivated by a thin dielectric layer which prevented inter‐diffusion of Cd and Cu at the top interface, thereby improving the junction quality. Application of the top passivation layers resulted in substantial improvement in V _oc and FF, thereby achieving the V _oc deficit of 0.542 V which is the record value among reported CZTSe solar cells. Copyright © 2017 John Wiley & Sons, Ltd.     

Synthesis and photovoltaic properties of D-A copolymers based on alkylthio-thiophene or alkylthio-selenophene-BDT donor unit and DPP acceptor unit

Two new 2D-conjugated D-A copolymers, PBDTT-S-DPP and PBDTSe-S-DPP, based on benzodithiophene (BDT) donor unit with alkylthio-thiophene or alkylthio-selenophene conjugated side chains and 2,5-bis(2-butyloctyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (DPP) acceptor unit, were synthesized for the application as donor materials in polymer solar cells (PSCs). The two polymers were characterized by absorption spectroscopy, cyclic voltammetry, thermogravimetric analysis, theoretical calculation with density functional theory, X-ray diffraction and photovoltaic measurements. The results show that the alkylthio-thiophene/selenophene side groups on BDT unit and intramolecular hydrogen bonding interaction in DPP acceptor unit play important roles in affecting the absorption, HOMO energy levels, molecular planarity and the crystallinity of the polymers. The PSCs based on PBDTT-S-DPP or PBDTSe-S-DPP as donor and PC₇₁BM as acceptor demonstrate power conversion efficiency (PCE) of 5.62% and 5.01%, with relatively higher V_oc of 0.79 V and 0.76 V, respectively.