Polymer Solar Cells with Efficiency >10% Enabled via a Facile Solution‐Processed Al‐Doped ZnO Electron Transporting Layer

A facile and low‐temperature (125 °C) solution‐processed Al‐doped ZnO (AZO) buffer layer functioning very effectively as electron accepting/hole blocking layer for a wide range of polymer:fullerene bulk heterojunction systems, yielding power conversion efficiency in excess of 10% (8%) on glass (plastic) substrates is described. The ammonia‐treatment of the aqueous AZO nanoparticle solution produces compact, crystalline, and smooth thin films, which retain the aluminum doping, and eliminates/reduces the native defects by nitrogen incorporation, making them good electron transporters and energetically matched with the fullerene acceptor. It is demonstrated that highly efficient solar cells can be achieved without the need for additional surface chemical modifications of the buffer layer, which is a common requirement for many metal oxide buffer layers to yield efficient solar cells. Also highly efficient solar cells are achieved with thick AZO films (>50 nm), highlighting the suitability of this material for roll‐to‐roll coating. Preliminary results on the applicability of AZO as electron injection layer in F8BT‐based polymer light emitting diode are also presented.     

Layer‐by‐Layer Solution‐Processed Low‐Bandgap Polymer‐PC61BM Solar Cells with High Efficiency

Polymer solar cells (PSCs) are fabricated without solvent additives using a low‐bandgap polymer, PBDTTT‐C‐T, as the donor and [6,6]‐phenyl‐C61‐butyric‐acid‐methyl‐ester (PC₆₁BM) as the acceptor. Donor‐acceptor blend and layer‐by‐layer (LL) solution process are used to form active layers. Relative to the blend devices, the LL devices exhibit stronger absorption, better vertical phase separation, higher hole and electron mobilities, and better charge extraction at correct electrodes. As a result, after thermal annealing the LL devices exhibit an average power conversion efficiency (PCE) of 6.86%, which is much higher than that of the blend devices (4.31%). The best PCE of the LL devices is 7.13%, which is the highest reported for LL processed PSCs and among the highest reported for PC₆₁BM‐based single‐junction PSCs.     

Fully Solution‐Processed TCO‐Free Semitransparent Perovskite Solar Cells for Tandem and Flexible Applications

Semitransparent perovskite solar cells (st‐PSCs) have received remarkable interest in recent years because of their great potential in applications for solar window, tandem solar cells, and flexible photovoltaics. However, all reported st‐PSCs require expensive transparent conducting oxides (TCOs) or metal‐based thin films made by vacuum deposition, which is not cost effective for large‐scale fabrication: the cost of TCOs is estimated to occupy ≈75% of the manufacturing cost of PSCs. To address this critical challenge, this study reports a low‐temperature and vacuum‐free strategy for the fabrication of highly efficient TCO‐free st‐PSCs. The TCO‐free st‐PSC on glass exhibits 13.9% power conversion efficiency (PCE), and the four‐terminal tandem cell made with the st‐PSC top cell and c‐Si bottom cell shows an overall PCE of 19.2%. Due to the low processing temperature, the fabrication of flexible st‐PSCs is demonstrated on polyethylene terephthalate and polyimide, which show excellent stability under repeated bending or even crumbing.     

Overcoming Electrode‐Induced Losses in Organic Solar Cells by Tailoring a Quasi‐Ohmic Contact to Fullerenes via Solution‐Processed Alkali Hydroxide Layers

It is shown that the performance of inverted organic solar cells can be significantly improved by facilitating the formation of a quasi‐ohmic contact via solution‐processed alkali hydroxide (AOH) interlayers on top of n‐type metal oxide (aluminum zinc oxide, AZO, and zinc oxide, ZnO) layers. AOHs significantly reduce the work function of metal oxides, and are further proven to effectively passivate defect states in these metal oxides. The interfacial energetics of these electron collecting contacts with a prototypical electron acceptor (C₆₀) are investigated to reveal the presence of a large interface dipole and a new interface state between the Fermi energy and the C₆₀ highest occupied molecular orbital for AOH‐modified AZO contacts. These novel interfacial gap states are a result of ground‐state electron transfer from the metal hydroxide‐functionalized AZO contact to the adsorbed molecules, which are hypothesized to be electronically hybridized with the contact. These interface states tail all the way to the Fermi energy, providing for a highly n‐doped (metal‐like) interfacial molecular layer. Furthermore, the strong “light‐soaking” effect is no longer observed in devices with a AOH interface.     

An Efficient Solution‐Processed Intermediate Layer for Facilitating Fabrication of Organic Multi‐Junction Solar Cells

Photovoltaic tandem technology has the potential to boost the power conversion efficiency of organic photovoltaic devices. Here, a reliable and efficient fully solution‐processed intermediate layer (IML) consisting of ZnO and neutralized poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is demonstrated for series‐connected multi‐junction organic solar cells (OSCs). Drying at 80 °C in air is sufficient for this solution‐processed IML to obtain excellent functionality and reliability, which allow the use of most of high performance donor materials in the tandem structure. An open circuit voltage (V_oc) of 0.56 V is obtained for single‐junction OSCs based on a low band‐gap polymer, while multi‐junction OSCs based on the same absorber material deliver promising fill factor values along with fully additive V_oc as the number of junctions increase. Optical and electrical simulations, which are reliable and promising guidelines for the design and investigation of multi‐junction OSCs, are discussed. The outcome of optical and electrical simulations is in excellent agreement with the experimental data, indicating the outstanding efficiency and functionality of this solution‐processed IML. The demonstration of this efficient, solution‐processed IML represents a convenient way for facilitating fabrication of multi‐junction OSCs to achieve high power conversion efficiency.     

Low‐Temperature Solution‐Processed CuCrO2 Hole‐Transporting Layer for Efficient and Photostable Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PVSCs) have become the front‐running photovoltaic technology nowadays and are expected to profoundly impact society in the near future. However, their practical applications are currently hampered by the challenges of realizing high performance and long‐term stability simultaneously. Herein, the development of inverted PVSCs is reported based on low temperature solution‐processed CuCrO₂ nanocrystals as a hole‐transporting layer (HTL), to replace the extensively studied NiO_x counterpart due to its suitable electronic structure and charge carrier transporting properties. A ≈45 nm thick compact CuCrO₂ layer is incorporated into an inverted planar configuration of indium tin oxides (ITO)/c‐CuCrO₂/perovskite/[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM)/bathocuproine (BCP)/Ag, to result in the high steady‐state power conversion efficiency of 19.0% versus 17.1% for the typical low temperature solution‐processed NiO_x‐based devices. More importantly, the optimized CuCrO₂‐based device exhibits a much enhanced photostability than the reference device due to the greater UV light‐harvesting of the CuCrO₂ layer, which can efficiently prevent the perovskite film from intense UV light exposure to avoid associated degradation. The results demonstrate the promising potential of CuCrO₂ nanocrystals as an efficient HTL for realizing high‐performance and photostable inverted PVSCs.     

Interface Design to Improve the Performance and Stability of Solution‐Processed Small‐Molecule Conventional Solar Cells

A systematic study on the effect of various cathode buffer layers on the performance and stability of solution‐processed small‐molecule organic solar cells (SMOSCs) based on tris{4‐[5‐(1,1‐dicyanobut‐1‐en‐2‐yl)‐2,2‐bithiophen‐5‐yl]phenyl}amine (N(Ph‐2T‐DCN‐Et)₃):6,6‐phenyl‐C71‐butyric acid methyl ester (N(Ph‐2T‐DCN‐Et)₃:PC₇₀BM) is presented. The power conversion efficiency (PCE) in these systems can be significantly improved from approximately 4% to 5.16% by inserting a metal oxide (ZnO) layer between the active layer and the Al cathode instead of an air‐sensitive Ba or Ca layer. However, the low work‐function Al cathode is susceptible to chemical oxidation in the atmosphere. Here, an amine group functionalized fullerene complex (DMAPA‐C₆₀) is inserted as a cathode buffer layer to successfully modify the interface towards ZnO/Ag and active layer/Ag functionality. For devices with ZnO/DMAPA‐C₆₀/Ag and DMAPA‐C₆₀/Ag cathodes the PCEs are improved from 2.75% to 4.31% and to 5.40%, respectively, compared to a ZnO/Ag device. Recombination mechanisms and stability aspects of devices with various cathodes are also investigated. The significant improvement in device performance and stability and the simplicity of fabrication by solution processing suggest this DMAPA‐C₆₀‐based interface as a promising and practical pathway for developing efficient, stable, and roll‐to‐roll processable SMOSCs.     

Non‐Fullerene Acceptor‐Based Bulk Heterojunction Polymer Solar Cells: Engineering the Nanomorphology via Processing Additives

The performance of bulk heterojunction solar cells made from blends of a non‐fullerene acceptor, N,N′‐bis(2‐ethylhexyl)‐2,6‐bis(5″‐hexyl‐[2,2′;5′,2″]terthiophen‐5yl)‐1,4,5,8‐naphthalene diimide (NDI‐3TH), and poly(3‐hexylthiophene) (P3HT) donor is enhanced 10‐fold by using a processing additive in conjunction with an electron‐blocking and a hole‐blocking buffer layers. The power conversion efficiency of P3HT:NDI‐3TH solar cells improves from 0.14% to 1.5% by using a processing additive (1,8‐diiodooctane) at an optimum concentration of 0.2 vol%, which is far below the 2‐3 vol% optimum concentrations found in polymer/fullerene systems. TEM and AFM imaging show that the size and connectivity of the NDI‐3TH domains in the phase‐separated P3HT:NDI‐3TH blends vary strongly with the concentration of the processing additive. These results demonstrate, for the first time, that processing additives can be effective in the optimization of the morphology and performance of bulk heterojunction polymer solar cells based on non‐fullerene acceptors.     

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

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

Efficient Solution‐Processed Bulk Heterojunction Perovskite Hybrid Solar Cells

Efficient conventional bulk heterojunction (BHJ) perovskite hybrid solar cells (pero‐HSCs) solution‐processed from a composite of CH₃NH₃PbI₃ mixed with PC₆₁BM ([6,6]‐phenyl‐C61‐butyric acid methyl ester), where CH₃NH₃PbI₃ acts as the electron donor and PC₆₁BM acts as the electron acceptor, are reported for the first time. The efficiency of 12.78% is twofold enhancement in comparison with the conventional planar heterojunction pero‐HSCs (6.90%) fabricated by pristine CH₃NH₃PbI₃. The BHJ pero‐HSCs are further optimized by using PC₆₁BM/TiO₂ bi‐electron‐extraction‐layer (EEL), which are both solution‐processed and then followed with low‐temperature thermal annealing. Due to higher electrical conductivity of PC₆₁BM over that of TiO₂, an efficiency of 14.98%, the highest reported efficiency for the pero‐HSCs without incorporating high‐temperature‐processed mesoporous TiO₂ and Al₂O₃ as the EEL and insulating scaffold, is observed from PC₆₁BM modified BHJ pero‐HSCs. Thus, the findings provide a simple way to approach high efficiency low‐cost pero‐HSCs.