Effects of Na and MoS2 on Cu2ZnSnS4 thin‐film solar cell

Cu₂ZnSnS₄ (CZTS)‐based materials have a useful band gap and a high absorption coefficient; however, their power conversion efficiency is low compared with that of CdTe and Cu(In,Ga)Se₂‐based solar cells. Two of the factors that strongly affect CZTS solar cell characteristics are the MoS₂ layer and the presence of defects. In this study, Mo back‐contact layers were annealed to control MoS₂ layer formation and the Na content in the Mo layer before the absorber precursor layer was deposited. The increase in oxygen content in the Mo layer suppressed MoS₂ layer formation. In addition, the increase in Na diffusion during the initial stage of the absorber precursor deposition decreased the defect density in the absorber layer and in the absorber–buffer interface. These results were verified through measurements of the external quantum efficiency, the temperature dependence of the open‐circuit voltage (V_OC), and admittance spectra. The current densities (J_SC) and V_OC, as well as the power conversion efficiencies, improved as the annealing temperature of the Mo layer increased, which suggests that CZTS solar cell characteristics can be improved by suppressing MoS₂ layer formation and increasing Na content in the Mo layer before deposition of the absorber precursor layer. Copyright © 2014 John Wiley & Sons, Ltd.     

Effective Ligand Engineering of the Cu2ZnSnS4 Nanocrystal Surface for Increasing Hole Transport Efficiency in Perovskite Solar Cells

Effective engineering of surface ligands in semiconductor nanocrystals can facilitate the electronic interaction between the individual nanocrystals, making them promising for low‐cost optoelectronic applications. Here, the use of high purity Cu₂ZnSnS₄ (CZTS) nanocrystals as the photoactive layer and hole‐transporting material is reported in low‐temperature solution‐processed solar cells. The high purity CZTS nanocrystals are prepared by engineering the surface ligands of CZTS nanocrystals, capped originally with the long‐chain organic ligand oleylamine. After ligand removal, CZTS nanocrystals show substantial improvement in photoconductivity and mobility, displaying also an appreciable photoresponse in a simple heterojunction solar cell architecture. More notably, CZTS nanocrystals exhibit excellent hole‐transporting properties as interface layer in perovskite solar cells, yielding power conversion efficiency (PCE) of 15.4% with excellent fill factor (FF) of 81%. These findings underscore the importance of removing undesired surface ligands in nanocrystalline optoelectronic devices, and demonstrate the great potential of CZTS nanocrystals as both active and passive material for the realization of low‐cost efficient solar cells.     

CuSCN Modified Back Contacts for High Performance CZTSSe Solar Cells

Optimization of the back contact interface is crucial for improving the performance of Cu₂ZnSnS₄ (CZTS) thin film solar cells. In this paper, self-depleted CuSCN is deployed as an intermediate layer at the Mo/CZTS interface to improve the quality of the back contact. This CuSCN layer, obtained via aqueous solution processing, reduces the thickness of Mo(S,Se)₂ and eliminates multi-layer crystallization of the absorber by suppressing the undesirable reaction between Mo and Se during the selenization process. By regulating the selenium infiltration into the CZTS precursor films during the selenization process, highly crystalline, single-layer Cu₂ZnSn(S,Se)₄ (CZTSSe) absorber layers are realized. The single-layer CZTSSe absorber exhibits reduced carrier recombination, enhanced carrier density and increased work function. The improved back contact and absorber layer enables 11.1% power-conversion-efficiency to be achieved.     

Improving the photovoltaic performance of co‐evaporated Cu2ZnSnS4 thin‐film solar cells by incorporation of sodium from NaF layers

We have investigated the influence of sodium (Na) on the properties of co‐evaporated Cu₂ZnSnS₄ (CZTS) layer microstructures and solar cells. The photovoltaic performance and diode properties were improved by incorporating Na from NaF layers into the CZTS layers, while Na had a negligible effect on the microstructural properties of the layer. The best cell fabricated by using an optimal CZTS layer (Cu/(Zn + Sn) = 0.70, Zn/Sn = 1.8) yielded an active area efficiency of 5.23%. The analysis of device properties suggests that charge‐carrier recombination at CZTS/CdS interface is suppressed by intentional Na incorporation from NaF layers. Copyright © 2016 John Wiley & Sons, Ltd.     

Cu2ZnSnS4 photovoltaic cell with improved efficiency fabricated by high‐temperature annealing after CdS buffer‐layer deposition

To improve the photovoltaic properties of Cu₂ZnSnS₄ (CZTS) cells, we investigated the effect of both the thickness of the deposited CdS layers and the post‐annealing temperature following CdS deposition on the photovoltaic properties of CZTS cells using a two‐layer CZTS structure. By depositing a thin CdS layer (40 nm) followed by high temperature annealing (603 K), we observed a remarkable increase in the short‐circuit current density because of the enhancement of the external quantum efficiency in the wavelength range of 400–800 nm. The best CZTS cell exhibited a conversion efficiency of 9.4% in the active area (9.1% in the designated area). In addition, we also fabricated a CZTS cell with open‐circuit voltage of 0.80 V by appropriately tuning the composition of the CZTS layers. Copyright © 2016 John Wiley & Sons, Ltd.     

Solid‐State Photovoltaic Thin Films using TiO2, Organic Dyes,and Layer‐by‐Layer Polyelectrolyte Nanocomposites

We report photovoltaic devices consisting of patterned TiO₂, porphyrin dyes, and layer‐by‐layer (LBL) polyelectrolyte multilayer/oligoethylene glycol dicarboxylic acid (OEGDA) composite films. A composite polyelectrolyte LBL/OEGDA film was fabricated by formation of an alternating multilayer of linear polyethyleneimine (LPEI) and polyacrylic acid (PAA), followed by immersion of the LBL film into an OEGDA aqueous solution. The ionic conductivity attained in this LBL LPEI/PAA and OEGDA composite film was approximately 10^–5 S cm^–1 at room temperature and humidity. Investigations of dye‐sensitized photovoltaic devices constructed with the LBL (LPEI/PAA)/OEGDA composite films, TiO₂, and four types of porphyrin dyes resulted in optimization of the dye molecule and its orientation at the interface with the ionically conductive composite. The photocurrent value of photovoltaic devices constructed with the composite LBL/OEGDA film from illumination of a xenon white light source exhibited a nearly 1.5 times enhancement over the device without OEGDA. This enhancement of the photocurrent was due to the high room‐temperature ionic conductivity of the multilayer composite film. Further marked improvements of the photovoltaic performance were achieved by patterning the TiO₂ electrode using polymer stamping as a template for TiO₂ deposition. The device with patterned TiO₂ electrodes exhibited almost 10 times larger conversion efficiencies than a similar device without patterning.     

Interface Engineering for Both Cathode and Anode Enables Low‐Cost Highly Efficient Solution‐Processed CdTe Nanocrystal Solar Cells

Low‐cost solution‐processed CdTe nanocrystal (NC) solar cells always suffer from a high interface energy barrier and unbalanced hole/electron transport as well as anisotropic atom diffusion on the CdTe surface due to the limited amount of hole/electron interface materials or the difficulty in interface processing. In this work, a novel strategy is first adopted with gradient electron transport layer (CdS/CdSe) modification in the cathode and a new crosslinkable hole transport polymer (P‐TPA) implantation in the anode. The carrier recombination at interfaces is greatly decreased and thus the carrier collection is increased. Moreover, the light harvesting is improved both in short and long wavelength regions, making J_sc and V_oc increase simultaneously. A champion solar cell shows a very high power conversion efficiency of 9.2% and an outstanding J_sc of 25.31 mA cm^?2, which are among the highest values for all solution‐processed CdTe NC solar cells with a superstrate structure, and the latter value is even higher than that of traditional thick CdTe thin‐film solar cells (2 µm) via the high temperature close space sublimation method. This work demonstrates that facile surface modifications in both the cathode and anode with stepped extraction and organic–inorganic hybridization are very promising in constructing next‐generation highly efficient NC photovoltaic devices.     

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.     

Locating the electrical junctions in Cu(In,Ga)Se2 and Cu2ZnSnSe4 solar cells by scanning capacitance spectroscopy

We determined the electrical junction (EJ) locations in Cu(In,Ga)Se₂ (CIGS) and Cu₂ZnSnSe₄ (CZTS) solar cells with ~20‐nm accuracy by developing scanning capacitance spectroscopy (SCS) applicable to the thin‐film devices. Cross‐sectional sample preparation for the SCS measurement was developed by high‐energy ion milling at room temperature for polishing the cross section to make it flat, followed by low‐energy ion milling at liquid nitrogen temperature for removing the damaged layer and subsequent annealing for growing a native oxide layer. The SCS shows distinct p‐type, transitional, and n‐type spectra across the devices, and the spectral features change rapidly with location in the depletion region, which results in determining the EJ with ~20‐nm resolution. We found an n‐type CIGS in the region next to the CIGS/CdS interface; thus, the cell is a homojunction. The EJ is ~40 nm from the interface on the CIGS side. In contrast, such an n‐type CZTS was not found in the CZTS/CdS cells. The EJ is ~20 nm from the CZTS/CdS interface, which is consistent with asymmetrical carrier concentrations of the p‐CZTS and n‐CdS in a heterojunction cell. Our results of unambiguously determination of the junction locations contribute significantly to understanding the large open‐circuit voltage difference between CIGS and CZTS. Copyright © 2016 John Wiley & Sons, Ltd.     

Chemically Controlled Reversible and Irreversible Extraction Barriers Via Stable Interface Modification of Zinc Oxide Electron Collection Layer in Polycarbazole‐based Organic Solar Cells

A spin‐cast method is presented for the formation of phosphonic acid functionalized small molecule layers on solution‐processed ZnO substrates for use as electron collecting interlayers in organic photovoltaics. Phosphonic acid interlayers modify the ZnO work function and the charge carrier injection barrier at its interface, resulting in systematic control of V _OC in inverted bulk heterojunction solar cells. Surface modification is shown to moderate the need for UV light‐soaking of the ZnO contact layers. Lifetime studies (30 days) indicate stable and improved OPV performance over the unmodified ZnO contact, which show significant increases in charge extraction barriers and series resistance. Results suggest that enhanced stability using small molecule modifiers is due to partial passivation of the oxide surface to molecular oxygen adsorption. Surface passivation while maintaining work function control of a selective interlayer can be employed to improve net efficiency and lifetime of organic photovoltaic devices. The modified cathode work function modulates V _OC via static energetic barriers and modulates contact conductivity by creating reversible and irreversible S‐shape current‐voltage characteristics as a result of kinetic barriers to charge transport.     

Hybrid Solar Cells from a Blend of Poly(3‐hexylthiophene) and Ligand‐Capped TiO2 Nanorods

Hybrid bulk heterojunction solar cells based on nanocrystalline TiO₂ (nc‐TiO₂) nanorods capped with trioctylphosphine oxide (TOPO) and regioregular poly(3‐hexylthiophene) (P3HT) are processed from solution and characterized in order to relate the device function (optical absorption, charge separation, and transport and photovoltaic properties) to active‐layer properties and device parameters. Annealing the blend films is found to greatly improve the polymer–metal oxide interaction at the nc‐TiO₂/P3HT interface, resulting in a six‐fold increase of the charge separation yield and improved photovoltaic device performance under simulated solar illumination. In addition, the influence of the organic ligand at the nc‐TiO₂ particle surface is found to be crucial for charge separation. Ligand‐exchange procedures applied on the TOPO‐capped nc‐TiO₂ nanorods with an amphiphilic ruthenium‐based dye are found to further improve the charge‐separation yield at the polymer–nanocrystal interface. However, the poor photocurrents generated in the hybrid blend devices, before and after ligand exchange, suggest that transport within or between nanoparticles limits performance. By comparison with other donor–acceptor bulk heterojunction systems, we conclude that charge transport in the nc‐TiO₂:P3HT blend films is limited by the presence of an intrinsic trap distribution mainly associated with the nc‐TiO₂ particles.     

Self‐Doping Fullerene Electrolyte‐Based Electron Transport Layer for All‐Room‐Temperature‐Processed High‐Performance Flexible Polymer Solar Cells

To achieve high‐performance large‐area flexible polymer solar cells (PSCs), one of the challenges is to develop new interface materials that possess a thermal‐annealing‐free process and thickness‐insensitive photovoltaic properties. Here, an n‐type self‐doping fullerene electrolyte, named PCBB‐3N‐3I, is developed as electron transporting layer (ETL) for the application in PSCs. PCBB‐3N‐3I ETL can be processed at room temperature, and shows excellent orthogonal solvent processability, substantially improved conductivity, and appropriate energy levels. PCBB‐3N‐3I ETL also functions as light‐harvesting acceptor in a bilayer solar cell, contributing to the overall device performance. As a result, the PCBB‐3N‐3I ETL‐based inverted PSCs with a PTB7‐Th:PC₇₁BM photoactive layer demonstrate an enhanced power conversion efficiency (PCE) of 10.62% for rigid and 10.04% for flexible devices. Moreover, the device avoids a thermal annealing process and the photovoltaic properties are insensitive to the thickness of PCBB‐3N‐3I ETL, yielding a PCE of 9.32% for the device with thick PCBB‐3N‐3I ETL (61 nm). To the best of one's knowledge, the above performance yields the highest efficiencies for the flexible PSCs and thick ETL‐based PSCs reported so far. Importantly, the flexible PSCs with PCBB‐3N‐3I ETL also show robust bending durability that could pave the way for the future development of high‐performance flexible solar cells.     

High‐efficiency Cu2ZnSn(S,Se)4 solar cells fabricated through a low‐cost solution process and a two‐step heat treatment

A method for fabricating high‐efficiency Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is presented, and it is based on a non‐explosive, low‐cost, and simple solution process followed by a two‐step heat treatment. 2‐Methoxyethanol was used as a solvent, and Cu, Zn, Sn, chloride salts, and thiourea were used as solutes. A CZTSSe absorber was prepared by sulfurising and then selenising an as‐coated Cu₂ZnSnS₄ (CZTS) film. Sulfurisation in a sulfur vapour filled furnace for a long time (2 h) enhanced the crystallisation of the as‐coated CZTS film and improved the stability of the CZTS precursor, and selenisation promoted further grain growth to yield a void‐free CZTSSe film. Segregation of Cu and S at the grain boundaries, the absence of a fine‐grain bottom layer, and the large grain size of the CZTSSe absorber were the main factors that enhanced the grain‐to‐grain transport of carriers and consequently the short‐circuit current (J_sc ) and efficiency. The efficiency of the CZTS solar cell was 5.0%, which increased to 10.1% after selenisation. For the 10.1% CZTSSe solar cell, the external quantum efficiency was approximately 80%, the open‐circuit voltage was 450 mV, the short‐circuit current was 36.5 mA/cm², and the fill factor was 61.9%. Copyright © 2016 John Wiley & Sons, Ltd.     

Rapid annealing of reactively sputtered precursors for Cu2ZnSnS4 solar cells

Cu₂ZnSnS₄ (CZTS) is a promising thin‐film absorber material that presents some interesting challenges in fabrication when compared with Cu(In,Ga)Se₂. We introduce a two‐step process for fabrication of CZTS films, involving reactive sputtering of a Cu‐Zn‐Sn‐S precursor followed by rapid annealing. X‐ray diffraction and Raman measurements of the sputtered precursor suggest that it is in a disordered, metastable CZTS phase, similar to the high‐temperature cubic modification reported for CZTS. A few minutes of annealing at 550 °C are sufficient to produce crystalline CZTS films with grain sizes in the micrometer range. The first reported device using this approach has an AM1.5 efficiency of 4.6%, with J_sc and V_oc both appearing to be limited by interface recombination. Copyright © 2012 John Wiley & Sons, Ltd.     

Low‐Work‐Function Surface Formed by Solution‐Processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate

Nanostructured layers of Cs₂CO₃ are shown to function very effectively as cathodes in organic electronic devices because of their good electron‐injection capabilities. Here, we report a comprehensive study of the origin of the low work function of nanostructured layers of Cs₂CO₃ prepared by solution deposition and thermal evaporation. The nanoscale Cs₂CO₃ layers are probed by various characterization methods including current–voltage (I–V) measurements, photovoltaic studies, X‐ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), and impedance spectroscopy. It is found that thermally evaporated Cs₂CO₃ decomposes into CsO₂ and cesium suboxides. The cesium suboxides dope CsO₂, yielding a heavily doped n‐type semiconductor with an intrinsically low work function. As a result, devices fabricated using thermally evaporated Cs₂CO₃ are relatively insensitive to the choice of the cathode metal. The reaction of thermally evaporated Cs₂CO₃ with Al can further reduce the work function to 2.1 eV by forming an Al–O–Cs complex. Solution‐processed Cs₂CO₃ also reduces the work function of Au substrates from 5.1 to 3.5 eV. However, devices prepared using solution‐processed Cs₂CO₃ exhibit high efficiency only if a reactive metal such as Al or Ca is used as the cathode metal. A strong chemical reaction occurs between spin‐coated Cs₂CO₃ and thermally evaporated Al. An Al–O—Cs complex is formed as a result of this chemical reaction at the interface, and this layer significantly reduces the work function of the cathode. Finally, impedance spectroscopy results prove that this layer is highly conductive.     

Enhanced Omnidirectional Photovoltaic Performance of Solar Cells Using Multiple‐Discrete‐Layer Tailored‐ and Low‐Refractive Index Anti‐Reflection Coatings

An optimized four‐layer tailored‐ and low‐refractive index anti‐reflection (AR) coating on an inverted metamorphic (IMM) triple‐junction solar cell device is demonstrated. Due to an excellent refractive index matching with the ambient air by using tailored‐ and low‐refractive index nanoporous SiO₂ layers and owing to a multiple‐discrete‐layer design of the AR coating optimized by a genetic algorithm, such a four‐layer AR coating shows excellent broadband and omnidirectional AR characteristics and significantly enhances the omnidirectional photovoltaic performance of IMM solar cell devices. Comparing the photovoltaic performance of an IMM solar cell device with the four‐layer AR coating and an IMM solar cell with the conventional SiO₂/TiO₂ double layer AR coating, the four‐layer AR coating achieves an angle‐of‐incidence (AOI) averaged short‐circuit current density, J_SC, enhancement of 34.4%, whereas the conventional double layer AR coating only achieves an AOI‐averaged J_SC enhancement of 25.3%. The measured reflectance reduction and omnidirectional photovoltaic performance enhancement of the four‐layer AR coating are to our knowledge, the largest ever reported in the literature of solar cell devices.     

Progression of metalorganic chemical vapour‐deposited CdTe thin‐film PV devices towards modules

This paper reports important developments achieved with CdTe thin‐film photovoltaic devices produced using metalorganic chemical vapour deposition at atmospheric pressure. In particular, attention was paid to understand the enhancements in solar cell conversion efficiency, to develop the cell design, and assess scalability towards modules. Improvements in the device performance were achieved by optimising the high‐transparency window layer (Cd_0.3Zn_0.7S) and a device‐activation anneal. These increased the fill factor and open‐circuit voltage to 77 ± 1% and 785 ± 7 mV, respectively, compared with 69 ± 3% and 710 ± 10 mV for previous baseline devices with no anneal and thicker Cd_0.3Zn_0.7S. The enhancement in these parameters is associated with the two fold to three fold increase in the net acceptor density of CdTe upon air annealing and a decrease in the back contact barrier height from 0.24 ± 0.01 to 0.16 ± 0.02 eV. The optimum thickness of the window layer for maximum photocurrent was 150 nm. The cell size was scaled from 0.25 to 2 cm² in order to assess its impact on the device series resistance and fill factor. Finally, micro‐module devices utilising series‐connected 2‐cm² sub‐cells were fabricated using a combination of laser and mechanical scribing techniques. An initial module‐to‐cell efficiency ratio of 0.9 was demonstrated for a six‐cell module with the use of the improved device structure and processing. Prospects for CdTe photovoltaic modules grown by metalorganic chemical vapour deposition are commented on. Copyright © 2015 John Wiley & Sons, Ltd.     

Charge Photogeneration in Few‐Layer MoS2

The 2D semiconductor MoS₂ in its mono‐ and few‐layer form is expected to have a significant exciton binding energy of several 100 meV, suggesting excitons as the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating efficient charge carrier photogeneration. Here, modulation spectroscopy in the sub‐ps and ms time scales is used to study the photoexcitation dynamics in few‐layer MoS₂. The results suggest that the primary photoexcitations are excitons that efficiently dissociate into charges with a characteristic time of 700 fs. Based on these findings, simple suggestions for the design of efficient MoS₂ photovoltaic and photodetector devices are made.     

Fully High‐Temperature‐Processed SnO2 as Blocking Layer and Scaffold for Efficient,Stable, and Hysteresis‐Free Mesoporous Perovskite Solar Cells

Planar perovskite solar cells (PSCs) based on low‐temperature‐processed (LTP) SnO₂ have demonstrated excellent photovoltaic properties duo to the high electron mobility, wide bandgap, and suitable band energy alignment of LTP SnO₂. However, planar PSCs or mesoporous (mp) PSCs based on high‐temperature‐processed (HTP) SnO₂ show much degraded performance. Here, a new strategy with fully HTP Mg‐doped quantum dot SnO₂ as blocking layer (bl) and a quite thin SnO₂ nanoparticle as mp layer are developed. The performances of both planar and mp PSCs has been greatly improved. The use of Mg‐SnO₂ in planar PSCs yields a high‐stabilized power conversion efficiency (PCE) of close to 17%. The champion of mp cells exhibits hysteresis free and stable performance with a high‐stabilized PCE of 19.12%. The inclusion of thin mp SnO₂ in PSCs not only plays a role of an energy bridge, facilitating electrons transfer from perovskite to SnO₂ bl, but also enhances the contact area of SnO₂ with perovskite absorber. Impedance analysis suggests that the thin mp layer is an “active scaffold” selectively collecting electrons from perovskite and can eliminate hysteresis and effectively suppress recombination. This is an inspiring advance toward high‐performance PSCs with HTP mp SnO₂.     

Cover Picture: Low‐Work‐Function Surface Formed by Solution‐Processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate (Adv. Funct. Mater. 12/2007)

The cover shows the structure of an efficient polymer light emitting diode (PLED) and its energy diagram at the interface between aluminum (Al) and a Cs₂CO₃ interfacial layer. It reveals the origin of enhanced electron injection from the Al electrode due to the low work function of a thermally evaporated Cs₂CO₃ layer, as reported on p. 1966 by Jinsong Huang, Zhen Xu, and Yang Yang. Pictures of the white‐ and red‐emitting PLEDs are also shown. Nanostructured layers of Cs₂CO₃ are shown to function very effectively as cathodes in organic electronic devices because of their good electron‐injection capabilities. Here, we report a comprehensive study of the origin of the low work function of nanostructured layers of Cs₂CO₃ prepared by solution deposition and thermal evaporation. The nanoscale Cs₂CO₃ layers are probed by various characterization methods including current–voltage (I–V) measurements, photovoltaic studies, X‐ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), and impedance spectroscopy. It is found that thermally evaporated Cs₂CO₃ decomposes into CsO₂ and cesium suboxides. The cesium suboxides dope CsO₂, yielding a heavily doped n‐type semiconductor with an intrinsically low work function. As a result, devices fabricated using thermally evaporated Cs₂CO₃ are relatively insensitive to the choice of the cathode metal. The reaction of thermally evaporated Cs₂CO₃ with Al can further reduce the work function to 2.1 eV by forming an Al–O–Cs complex. Solution‐processed Cs₂CO₃ also reduces the work function of Au substrates from 5.1 to 3.5 eV. However, devices prepared using solution‐processed Cs₂CO₃ exhibit high efficiency only if a reactive metal such as Al or Ca is used as the cathode metal. A strong chemical reaction occurs between spin‐coated Cs₂CO₃ and thermally evaporated Al. An Al–O—Cs complex is formed as a result of this chemical reaction at the interface, and this layer significantly reduces the work function of the cathode. Finally, impedance spectroscopy results prove that this layer is highly conductive.