Reduced Recombination Losses Research Articles

Probing energy losses from dye desorption in cobalt complex-based dye-sensitized solar cells.

Self-assembly of organic sensitizer layers in cobalt complex-based DSCs was studied to elucidate its role in reducing the loss of charge recombination. DSCs with various dye loadings were fabricated by dye desorption without the aid of basic solvent. The FT-IR and UV results indicate the deprotonation of the anchoring organic sensitizers, which influences the conduction band of TiO2 remarkably by changing the surface potential. Positive band edge shifts and a decrease of the recombination rate constant are demonstrated to be the main factors affecting energy loss at open circuit. In contrast, absorbed photon conversion efficiency (APCE) analyses illuminate the crucial role of the packing of the anchoring sensitizer in reducing recombination loss at short circuit. This is further supported by numerical simulations, which show that APCE is primarily dependent on the recombination rate constant rather than the band edge shift at short circuit. These results highlight the importance of self-assembly of sensitizers with insulating groups in retarding charge recombination by forming overlapping molecular layers.

Physics-Based Modeling and Experimental Study of Si-Doped InAs/GaAs Quantum Dot Solar Cells

This paper presents an experimental and theoretical study on the impact of doping and recombination mechanisms on quantum dot solar cells based on the InAs/GaAs system. Numerical simulations are built on a hybrid approach that includes the quantum features of the charge transfer processes between the nanostructured material and the bulk host material in a classical transport model of the macroscopic continuum. This allows gaining a detailed understanding of the several physical mechanisms affecting the photovoltaic conversion efficiency and provides a quantitatively accurate picture of real devices at a reasonable computational cost. Experimental results demonstrate that QD doping provides a remarkable increase of the solar cell open-circuit voltage, which is explained by the numerical simulations as the result of reduced recombination loss through quantum dots and defects.

Enhanced Performance of Perovskite Solar Cells with Zinc Chloride Additives.

Perovskite solar cells (PSCs) have attracted extensive attention due to their impressive photovoltaic performance. The quality of the perovskite layer is very critical to achieve high device performance. Here, we explore the partial substitution of PbI2 by ZnCl2 in the preparation of CH3NH3PbI3 and its effects on perovskite morphology, optical properties, and photovoltaic performance. Consequently, the device with 3% ZnCl2 shows great improvement in power conversion efficiency (PCE) from 16.4 to 18.2% compared to that of the control device. Moreover, the device is more stable than the control device, with only 7% degradation after aging for 30 days. These results are attributed to the increased grain size, improved film morphology, and reduced recombination loss after the partial substitution of PbI2 by ZnCl2 in the perovskite film. This work develops a new approach for morphology control through rational additives in the perovskite film, and paves the way toward further enhancing the device performances of PSCs including PCE and stability.

Microwave exposure as a fast and cost-effective alternative of oxygen plasma treatment of indium-tin oxide electrode for application in organic solar cells

Pre-treatment methods are commonly employed to clean as well as to modify electrode surfaces. Many previous reports suggest that modifying the surface properties of indium tin oxide (ITO) by oxygen plasma treatment is a crucial step for the fabrication of high performance organic solar cells. In this work, we propose a fast and cost-effective microwave exposure step for the modification of the surface properties of ITO anode electrodes used in organic solar cells. It is demonstrated that a short microwave exposure improves the hydrophilicity and reduces the roughness of the ITO surface, as revealed by contact angle and atomic force microscopy (AFM) measurements, respectively, leading to a better quality of the PEDOT:PSS film coated on top of it. Similar results were obtained with the commonly used oxygen plasma treatment of ITO suggesting that microwave exposure is an effective process for modifying the surface properties of ITO with the benefits of low-cost, easy and fast processing. In addition, the influence of the microwave exposure of ITO anode electrode on the performance of an organic solar cell based on the poly(3-hexylthiophene):[6,6]-phenyl C70 butyric acid methyl ester (P3HT:PC70BM) blend is investigated. The 71% efficiency enhancement obtained in the microwave annealed-ITO based device as compared to the device with the as-received ITO was mainly attributed to the improvement in the short circuit current (Jsc) and decreased leakage current caused by the reduced series and the increased shunt resistances and also by the higher charge generation efficiency, and the reduced recombination losses.

Surface fluorination of α-Fe2O3 using selectfluor for enhancement in photoelectrochemical properties

Fluorinated α-Fe2O3 nanostructures are synthesized via a facile hydrothermal route using Selectfluor™ (F-TEDA) as a fluorinating as well as growth directing agent. The addition of incrementally increasing amount of F-TEDA to Fe precursor under hydrothermal conditions resulted in preferential growth of α-Fe2O3 along (110) orientation with respect to (104) direction by ~ 35%, the former being important for enhanced charge transport. On increasing fluorination, the heirarchical dendritic-type α-Fe2O3 changes to a snow-flake type structure (F-TEDA-20%) anisotropically growing along the six directions however, at higher F-TEDA concentrations (≥ 30%), loosely held particulate aggregates are seen to be formed. The X-Ray Photoelectron Spectroscopy (XPS) suggest the maximum fluorinarion of α-Fe2O3 at 1.21at% in 30% F-TEDA. Further, optical absorption studies reveal reduction in optical band gap from 2.10eV in case of pristine to 1.95eV for fluorinated α-Fe2O3. A photoanode made by taking 20% fluorinated α-Fe2O3 in a ratio of 10:90 with respect to TiO2 (P-25) showed improved performance in dye sensitized solar cells with an increase in efficiency by ~16% in comparision to that of pristine Fe2O3 and TiO2. Furthermore, anode consisting of thin films of fluorinated α-Fe2O3 on FTO also exhibit enhanced current density on illumination of ~100W/m2. The increase in photoelectrochemical activity seems to be due to the combination of two factors namely preferential growth of α-Fe2O3 along (110) direction resulting in an improved charge transfer efficiency and reduced recombination losses due to the presence of fluorine.

How to achieve efficiencies exceeding 22% with multicrystalline n-type silicon solar cells

In this contribution, we demonstrate a route for efficiencies exceeding 22% with n-type multicrystalline (mc) silicon solar cells based on the TOPCon cell concept featuring a boron-diffused front side emitter and a full-area passivating rear contact. By applying a “high-performance” (HP) crystallization process with an adapted seed structure in order to obtain an optimized grain boundary area fraction, we reduce recombination losses in the HP mc-Si material to a minimum. We discuss the electrical properties of the optimized n-type HP mc-Si, which features very low material-related efficiency losses of approximately 0.5%abs and, thus, enables an efficiency potential of 22.6% with regard to a cell limit of 23.1% of the TOPCon cell concept adapted for multicrystalline silicon. Results at the device level reveal a record efficiency of 21.9%, which is the highest efficiency reported for a multicrystalline silicon solar cell. Finally, we discuss the deviations between the predicted efficiency potential and the solar cell results.

(Invited) Reflections on Rust: Iron Oxide Photoelectrodes for Solar Energy Conversion and Storage

Solar water splitting is a promising route for sustainable and scalable hydrogen production from renewable sources: water and sunlight. The hydrogen can be used as a clean fuel for refueling fuel cell electric vehicles with zero emissions. Hydrogen can also serve as a feedstock for the production of drop-in liquid fuels by CO2 hydrogenation, and ammonia via the Haber–Bosch process. The greatest challenges towards viable solar water splitting technology lay in the selection and design of photocatalytic materials and devices that harvest sunlight and split water efficiently and produce hydrogen at a competitive cost. To this end photocatalytic materials that are stable in alkaline or acidic aqueous solutions, absorb visible light, promote water oxidation or reduction reactions and comprise of abundant materials must be employed. Iron oxide (α-Fe2O3, hematite), aka rust, is one of few materials meeting these criteria, but its poor transport properties and fast charge recombination present challenges for efficient charge carrier generation, separation and collection. We explore innovative solutions to these challenges using ultrathin (20-30 nm) films on specular back reflectors. This simple optical cavity design effectively traps the light in otherwise nearly translucent ultrathin films, amplifying the intensity close to the surface wherein photogenerated charge carriers can reach the surface and split water before recombination takes place. This is the enabling key towards the development of high-efficiency iron oxide photoelectrodes whose structure and properties can be tailored by design. However, the design rules for these devices remain elusive because iron oxide behaves differently than conventional semiconductors such as silicon, probably due to the strong correlation between the electrons that give rise to the current and the ions that form the crystalline lattice. Our first goal is to discover the physico-chemical processes that limit the performance of iron oxide photoelectrode and devise design principles that enable overcoming these limitations through material design at the nanoscale that combines iron oxide together with selective underlayers and overlayers that separate positive and negative charge carriers, thereby reducing recombination losses and increasing efficiency. The second goal is to apply these design principles to construct high-efficiency solar water splitting devices. The third and final goal is to invent new device architectures and operation schemes that enable affordable solar hydrogen production in large-scale applications, closing the gap between basic research on lab-scale devices and a new solar energy technology that provides stable power on demand. Figure 1

22.4% bifacial n-PERT cells with Ni/Ag co-plated contacts and Voc ~691 mV

In this work we present bifacial cells with Ni/Ag co-plated contacts with average Voc values of 691±2 mV and efficiencies of 22.4±0.2 % (internal measurement using a lowly-reflective GridtouchTM contacting unit from Pasan). The Ni/Ag co-plating sequence used in this paper presents numerous advantages, most notably: non-contact batch processing and cell metallization costs potentially below 7 $ct./cell. In the first part of this paper, two different emitter passivation schemes are compared (SiO2/SiNx and Al2O3/SiNx) and we demonstrate that Al2O3/SiNx enables a ~17mV gain in Voc and ~0.3 mA/cm2 gain in jsc thanks to reduced recombination losses. In the second part of this paper, proof-of-concept glass/backsheet and glass/glass 1-cell laminates are fabricated using multi-wires interconnection and light I-V results are presented. Finally, we demonstrate that one of the first 1-cell glass/backsheet laminates with Ni/Ag co-plated contacts and multi-wires interconnection currently experiences less than 4%abs. loss in maximum power after 400 thermal cycles (-40 to +85°C).

Improved Stability of Polymer Solar Cells in Ambient Air via Atomic Layer Deposition of Ultrathin Dielectric Layers

Polymer solar cells have attracted tremendous interest in the highly competitive solar energy sector, due to the practical advantages they exhibit, such as being lightweight, flexible, and low cost, in stark contrast to traditional photovoltaic technologies. However, their successful commercialization is still hindered by issues related to device instability. Here, atomic layer deposition (ALD) is employed to deposit conformal ultrathin dielectrics, such as alumina (Al2O3) and zirconia (ZrO2), on top of ZnO electron extraction layers to address problems that arise from the defect‐rich nature of these layers. The deposition of dielectrics on ZnO significantly improves its interfacial electronic properties, manifested primarily with the decrease in the work function of ZnO and the concomitant reduction of the electron extraction barrier as well as the reduced recombination losses. Significant efficiency enhancement is obtained with the incorporation of six ALD cycles of Al2O3 into inverted devices, using photoactive layers, that consist of poly(3‐hexylthiophene):indene‐C60‐bisadduct or poly({4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl}{3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl] thieno[3,4‐b] thiophenediyl}):[6,6]‐phenyl‐C70‐butyric acid methyl ester. More importantly, upon performing lifetime studies (over a period of 350 h), a strong improvement in polymer solar cell stability is observed when using the ALD‐modified ZnO films.

High-Efficiency Nonfullerene Organic Solar Cells with a Parallel Tandem Configuration.

In this work, a highly efficient parallel connected tandem solar cell utilizing a nonfullerene acceptor is demonstrated. Guided by optical simulation, each of the active layer thicknesses of subcells are tuned to maximize its light trapping without spending intense effort to match photocurrent. Interestingly, a strong optical microcavity with dual oscillation centers is formed in a back subcell, which further enhances light absorption. The parallel tandem device shows an improved photon-to-electron response over the range between 450 and 800 nm, and a high short-circuit current density (J SC ) of 17.92 mA cm-2 . In addition, the subcells show high fill factors due to reduced recombination loss under diluted light intensity. These merits enable an overall power conversion efficiency (PCE) of >10% for this tandem cell, which represents a ≈15% enhancement compared to the optimal single-junction device. Further application of the designed parallel tandem configuration to more efficient single-junction cells enable a PCE of >11%, which is the highest efficiency among all parallel connected organic solar cells (OSCs). This work stresses the importance of employing a parallel tandem configuration for achieving efficient light harvesting in nonfullerene-based OSCs. It provides a useful strategy for exploring the ultimate performance of organic solar cells.

It Takes Two to Tango-Double-Layer Selective Contacts in Perovskite Solar Cells for Improved Device Performance and Reduced Hysteresis.

Solar cells made from inorganic-organic perovskites have gradually approached market requirements as their efficiency and stability have improved tremendously in recent years. Planar low-temperature processed perovskite solar cells are advantageous for possible large-scale production but are more prone to exhibiting photocurrent hysteresis, especially in the regular n-i-p structure. Here, a systematic characterization of different electron selective contacts with a variety of chemical and electrical properties in planar n-i-p devices processed below 180 °C is presented. The inorganic metal oxides TiO2 and SnO2, the organic fullerene derivatives C60, PCBM, and ICMA, as well as double-layers with a metal oxide/PCBM structure are used as electron transport materials (ETMs). Perovskite layers deposited atop the different ETMs with the herein applied fabrication method show a similar morphology according to scanning electron microscopy. Further, surface photovoltage spectroscopy measurements indicate comparable perovskite absorber qualities on all ETMs, except TiO2, which shows a more prominent influence of defect states. Transient photoluminescence studies together with current-voltage scans over a broad range of scan speeds reveal faster charge extraction, less pronounced hysteresis effects, and higher efficiencies for devices with fullerene compared to those with metal oxide ETMs. Beyond this, only double-layer ETM structures substantially diminish hysteresis effects for all performed scan speeds and strongly enhance the power conversion efficiency up to a champion stabilized value of 18.0%. The results indicate reduced recombination losses for a double-layer TiO2/PCBM contact design: First, a reduction of shunt paths through the fullerene to the ITO layer. Second, an improved hole blocking by the wide band gap metal oxide. Third, decreased transport losses due to an energetically more favorable contact, as implied by photoelectron spectroscopy measurements. The herein demonstrated improvements of multilayer selective contacts may serve as a general design guideline for perovskite solar cells.

Densely Packed Random Quarterpolymers Containing Two Donor and Two Acceptor Units: Controlling Absorption Ability and Molecular Interaction to Enable Enhanced Polymer Photovoltaic Devices

Dithienyldiketopyrrolopyrrole (DPP2T) and thieno[3,2‐b]thiophene (TT) building blocks, enabling a large intermolecular overlap through π–π stacking, into an amorphous‐like polymer composed of benzo(1,2‐b:4,5‐b′)dithiophene (BDT) and fluorinated thieno[3,4‐b]thiophene (QTT), are introduced. Herein, through the variation of relative compositions of DPP2T‐TT and BDT‐QTT in the polymer backbone, the synthesis and characterization of a series of condensed random 2D‐2A “quarterpolymers” with two reference alternating copolymers are reported. The best power conversion efficiency (PCE) of 9.45% is achieved for the optimum composition due to the synergistic effects such as improved photon absorption and reduced recombination loss, and optimized blend morphology via a change in the crystallinity and orientation of the blend films compared to the alternating copolymers. Moreover, by isolating higher molecular weight and narrower polydispersity fractions of the quarterpolymer via a marginal solvent‐soaking technique, the PCE is further boosted to 10.30%, which is among the highest PCE reported to date for random polymer‐based PSCs. Therefore, this simple 2D‐2A strategy, reported for the first time, should be extended to numerous quaterpolymer systems, greatly accelerating random polymer systems toward further improving PSCs.

Efficient Bulk Heterojunction CH3NH3PbI3-TiO2 Solar Cells with TiO2 Nanoparticles at Grain Boundaries of Perovskite by Multi-Cycle-Coating Strategy.

A novel bulk heterojunction (BHJ) perovskite solar cell (PSC), where the perovskite grains act as donor and the TiO2 nanoparticles act as acceptor, is reported. This efficient BHJ PSC was simply solution processed from a mixed precursor of CH3NH3PbI3 (MAPbI3) and TiO2 nanoparticles. With dissolution and recrystallization by multi-cycle-coating, a unique composite structure ranging from a MAPbI3-TiO2-dominated layer on the substrate side to a pure perovskite layer on the top side is formed, which is beneficial for the blocking of possible contact between TiO2 and the hole transport material at the interface. Scanning electron microscopy clearly shows that TiO2 nanoparticles accumulate along the grain boundaries (GBs) of perovskite. The TiO2 nanoparticles at the GBs quickly extract and reserve photogenerated electrons before they transport into the perovskite phase, as described in the multitrapping model, retarding the electron-hole recombination and reducing the energy loss, resulting in increased VOC and fill factor. Moreover, the pinning effect of the TiO2 nanoparticles at the GBs from the strong bindings between TiO2 and MAPbI3 suppresses massive ion migration along the GBs, leading to improved operational stability and diminished hysteresis. Photoluminescence (PL) quenching and PL decay confirm the efficient exciton dissociation on the heterointerface. Electrochemical impedance spectroscopy and open-circuit photovoltage decay measurements show the reduced recombination loss and improved carrier lifetime of the BHJ PSCs. This novel strategy of device design effectively combines the benefits of both planar and mesostructured architectures whilst avoiding their shortcomings, eventually leading to a high PCE of 17.42% under 1 Sun illumination. The newly proposed approach also provides a new way to fabricate a TiO2-containing perovskite active layer at a low temperature.

Reduction in Photocurrent Loss and Improvement in Performance of Single Junction Solar Cell Due to Multistep Grading of Hydrogenated Amorphous Silicon Germanium Active Layer

Single junction solar cells were fabricated with intrinsic hydrogenated amorphous silicon germanium (a-SiGe:H) as the active layer, that shows a 10% photovoltaic conversion efficiency. The a-SiGe:H active layer of the solar cells of type-A had constant band gap materials while that of type-B had a four step graded band gap by composition gradient (CG). The cells with composition gradient show an enhancement in fill factor and open circuit voltage (V oc) by 5% and 20 mV respectively, with respect to a cell without the graded band gap active layer. Such an enhanced device performance is attributed to reduction in recombination loss of photo-generated electron hole pairs. The effect of this photo-current loss was investigated in the electrical bias dependent external quantum efficiencies (EQE), illumination dependent current-voltage measurements and dark current-voltage characteristics. The device parameters like reverse saturation current density (J o), series resistance (R s) and diode quality factor (n) were also estimated. In comparison to the cell without the composition gradient, the EQE shows a reduced recombination loss across the whole wavelength range for the cell with the CG. Furthermore, the introduction of the CG results in a significantly increased shunt resistance from 720 to 1200 Ω.cm 2. The estimated n values of the cells under dark operating condition, decreases from 1.8 to 1.7 along with J o from 3 × 10 −7 down to 4.5 × 10 −8 A/cm 2 with CG, while the same parameters decreased from 3.73 to 3.06, 2.96 × 10 −6 to 3.05 × 10 −7 A/cm 2 respectively under AM1.5G insolation.

Surface Optimization of Random Pyramid Textured Silicon Substrates for Improving Heterojunction Solar Cells

Key steps in the fabrication of high-efficiency a-Si:H/c-Si heterojunction solar cells are the controlled pyramid texturing of the c-Si substrates to minimize reflection losses and the subsequent passivation by deposition of a high-quality a-Si:H layer to reduce recombination losses. This contribution reviews our recent results on the optimization of the wet-chemical texturing of crystalline Si wafers for the preparation of heterojunction solar cells with respect to low reflection losses, low recombination losses and long minority carrier lifetimes. It is demonstrated, that by joint optimization of both saw damage etch and texture etch the optical and electronic properties of the resulting pyramid morphology can be controlled. Effective surface passivation and thus long minority charge carrier lifetimes are achieved by deposition of intrinsic amorphous Si ((i) a-Si:H) layers. It is shown, that optimized (i) a-Si:H deposition parameters for planar Si (111) wafers can be transferred to a-Si:H layer deposition on random pyramid textured Si (100) wafers. Statistical analysis of the pyramid size distribution revealed that a low fraction of small pyramids leads to longer minority charge carrier lifetimes and, thus, a higher Voc potential for solar cells.

Carrier collection losses in interface passivated amorphous silicon thin-film solar cells

In silicon thin-film solar cells the interface between the i- and p-layer is the most critical. In the case of back diffusion of photogenerated minority carriers to the i/p-interface, recombination occurs mainly on the defect states at the interface. To suppress this effect and to reduce recombination losses, hydrogen plasma treatment (HPT) is usually applied. As an alternative to using state of the art HPT we apply an argon plasma treatment (APT) before the p-layer deposition in n-i-p solar cells. To study the effect of APT, several investigations were applied to compare the results with HPT and no plasma treatment at the interface. Carrier collection losses in resulting solar cells were examined with spectral response measurements with and without bias voltage. To investigate single layers, surface photovoltage and X-ray photoelectron spectroscopy (XPS) measurements were conducted. The results with APT at the i/p-interface show a beneficial contribution to the carrier collection compared with HPT and no plasma treatment. Therefore, it can be concluded that APT reduces the recombination centers at the interface. Further, we demonstrate that carrier collection losses of thin-film solar cells are significantly lower with APT.

In Russian

Photoelectrochemical properties have been studied of the electrodes based on the nanostructure containing TiO 2 nanotubes (NT–TiO 2 ) and CdSe films modified with graphene oxide (GO). Formation of TiO 2 nanotubes on titanium substrate increases the adhesion of deposited layer of polycrystalline CdSe. Such a modification of film by GO reduces the value of the cathode dark leakage current in an alkaline sulfide electrolyte, and the value of the open circuit voltage reaches 0.60 V. It has been determined the optimal amount of modifying agent GO ∼1%, which provides high photosensitivity electrodes. From the studies on the Raman spectra of polycrystalline CdSe films after annealing at 530°C where lines 2LO - and 3LO-phonons were observed, a conclusion is drawn on the quality of films. The reason for increasing the sensitivity of electrodes modified CdSe is an improved separation of electron-hole pairs in the space charge region (SCR) in contact with the electrolyte. Since the graphene structures are electron acceptors, so increasing the negative charge of the semiconductor surface in contact CdSe/GO, this leads to an increase in the surface electric field of the SCR. As a result of reduced recombination losses in the SCR, moreover, the GO particles can interact with the surface active sites which are recombination centers, so neutralizing the action of these centers. Composite photoanodes of NT-TiO2/CdSe/GO provide the evolution of hydrogen on platinum cathode as well as the cathode materials based on the reduced graphene oxide modified with nanoparticles of Pt, and multi-walled carbon nanotubes. The results obtained show that the investigated photoelectrodes are promising for use in the electrochemical systems for solar energy conversion.

Hybrid Perovskite/Perovskite Heterojunction Solar Cells.

Recently developed organic-inorganic hybrid perovskite solar cells combine low-cost fabrication and high power conversion efficiency. Advances in perovskite film optimization have led to an outstanding power conversion efficiency of more than 20%. Looking forward, shifting the focus toward new device architectures holds great potential to induce the next leap in device performance. Here, we demonstrate a perovskite/perovskite heterojunction solar cell. We developed a facile solution-based cation infiltration process to deposit layered perovskite (LPK) structures onto methylammonium lead iodide (MAPI) films. Grazing-incidence wide-angle X-ray scattering experiments were performed to gain insights into the crystallite orientation and the formation process of the perovskite bilayer. Our results show that the self-assembly of the LPK layer on top of an intact MAPI layer is accompanied by a reorganization of the perovskite interface. This leads to an enhancement of the open-circuit voltage and power conversion efficiency due to reduced recombination losses, as well as improved moisture stability in the resulting photovoltaic devices.

Influence of Blend Ratio and Processing Additive on Free Carrier Yield and Mobility in PTB7:PC71BM Photovoltaic Solar Cells.

Charge separation and extraction dynamics were investigated in high-performance bulk heterojunction solar cells made from the polymer PTB7 and the soluble fullerene PC71BM on a broad time scale from subpicosecond to microseconds using ultrafast optical probing of carrier drift and the integral-mode photocurrent measurements. We show that the short circuit current is determined by the separation of charge pairs into free carriers, which is strongly influenced by blend composition. This separation is found to be efficient in fullerene-rich blends where a high electron mobility of >0.1 cm2 V–1 s–1 is observed in the first 10 ps after excitation. Morphology optimization using the solvent additive 1,8-diiodooctane (DIO) doubles the charge pair separation efficiency and the short-circuit current. Carrier extraction at low internal electric field is slightly faster from the cells prepared with DIO, which can reduce recombination losses and enhance a fill factor.

Low-Temperature TiOx Compact Layer for Planar Heterojunction Perovskite Solar Cells.

Here, we demonstrate an effective low-temperature approach to fabricate a uniform and pinhole-free compact TiO2 layer for enhancing photovoltaic performance of perovskite solar cells. TiCl4 was used to modify TiO2 for efficient charge generation and significantly reduced recombination loss. We found that a TiO2 layer with an appropriate TiCl4 treatment possesses a smooth surface with full coverage of the conductive electrode. Further studies on charge carrier dynamics confirmed that the TiCl4 treatment improves the contact of the TiO2/perovskite interface, facilitating charge extraction and suppressing charge recombination. On the basis of the treatment, we improved the open circuit voltage from 1.01 V of the reference cell to 1.08 V, and achieved a power conversion efficiency of 16.4%.