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.     

An investigation of photocurrent loss due to reflectance and absorption in CdTe/CdS heterojunction solar cells

An attempt is made to understand, quantify, and reduce the reflectance and photocurrent loss in CdTe solar cells. Model calculations are performed to determine the optimum thicknesses of CdS and SnO₂ films and anti-reflection (AR) coating on glass that can minimize the reflectance and enhance the performance of CdTe/CdS/SnO₂/glass solar cells. Photocurrent loss due to absorption in CdS films is also calculated as a function of CdS thickness. It was found that the current loss due to reflectance ando absorption is more sensitive to the CdS film when its thickness falls below 1500Å. Model calculations show that reducing the CdS thickness from 1500 to 600Å increases short-circuit current density ( J_sc) by 3 mA/cm² because of reduced reflectance as well as absorption. Further decrease in CdS thickness below 600Å increases reflectance but results in higher J_sc, because current gain due to reduced absorption in thin CdS offsets the current loss due to higher reflectance. Model calculations also indicate that J_sc is not sensitive to SnO₂ thickness above 4000Å. Finally, an optimum thickness for single layer MgF₂ AR coating on glass was calculated to be 1100Å, which should provide an additional increase of 0.7 mA/cm² in J_sc. Some of these results are also experimentally validated in this paper.     

Reduced Efficiency Roll‐Off and Improved Stability of Mixed 2D/3D Perovskite Light Emitting Diodes by Balancing Charge Injection

Perovskite light emitting diodes (PeLEDs) have reached external quantum efficiencies (EQEs) over 21%. Their EQE, however, drops at increasing current densities (J) and their lifetime is still limited to just a few hours. The mechanisms leading to EQE roll‐off and device instability require thorough investigation. Here, improvement in EQE, EQE roll‐off, and lifetime of PeLEDs is demonstrated by tuning the balance of electron/hole transport into a mixed 2D/3D perovskite emissive layer. The mixed 2D/3D perovskite layer induces exciton confinement and beneficially influences the electron/hole distribution inside the perovskite layer. By tuning the electron injection to match the hole injection in such active layer, a nearly flat EQE for J = 0.1–200 mA cm^?2, a reduced EQE roll‐off until J = 250 mA cm^?2, and a half‐lifetime of ≈47 h at J = 10 mA cm^?2 is reached. A model is also proposed to explain these improvements that account for the spatial electron/hole distributions.     

Achievement of 17.9% efficiency in 30 × 30 cm2 Cu(In,Ga)(Se,S)2 solar cell sub‐module by sulfurization after selenization with Cd‐free buffer

We have achieved 17.9% efficiency in a 30 × 30 cm² Cu(In,Ga)(Se,S)₂ solar cell sub‐module prepared by selenization and sulfurization processes with a Cd‐free buffer. The development of an absorber layer, transparent conducting oxide window layer, and module design was the key focus. This permitted 1.8% higher efficiency than our last experimental result. The quantity and the injection time of the sodium were controlled, resulting in higher open circuit voltage (V_oc) and short circuit current (J_sc). In order to increase J_sc, we changed the thickness of the window layer. Boron‐doped zinc oxide was optimized for higher transmittance without reducing the fill factor. The uniformity of each layer was improved, and patterns were optimized for each module. Therefore, V_oc, J_sc, and FF could be theoretically improved on the reported results of, respectively, 20 mV, 2 mA/cm², and 1.4%. The module's efficiency was measured at the Korea Test Laboratory to compare with the data obtained in‐house. Various analyses were performed, including secondary ion mass spectroscopy, photoluminescence, quantum efficiency, solar simulator, and UV–vis spectrometry, to measure the cell's depth profile, carrier lifetime, external quantum efficiency, module efficiency, and transmittance, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

Novel Conjugated Organic Dyes for Efficient Dye‐Sensitized Solar Cells

Novel conjugated organic dyes that have N,N‐dimethylaniline (DMA) moieties as the electron donor and a cyanoacetic acid (CAA) moiety as the electron acceptor were developed for use in dye‐sensitized nanocrystalline‐TiO₂ solar cells (DSSCs). We attained a maximum solar‐energy‐to‐electricity conversion efficiency (η) of 6.8 % under AM 1.5 irradiation (100 mW cm^–2) with a DSSC based on 2‐cyano‐7,7‐bis(4‐dimethylamino‐phenyl)hepta‐2,4,6‐trienoic acid (NKX‐2569): short‐circuit photocurrent density (J_sc) = 12.9 mA cm^–2, open‐circuit voltage (V_oc) = 0.71 V, and fill factor (ff) = 0.74. The high performance of the solar cells indicated that highly efficient electron injection from the excited dyes to the conduction band of TiO₂ occurred. The experimental and calculated Fourier‐transform infrared (FT‐IR) absorption spectra clearly showed that these dyes were adsorbed on the TiO₂ surface with the carboxylate coordination form. A molecular‐orbital calculation indicated that the electron distribution moved from the DMA moiety to the CAA moiety by photoexcitation of the dye.     

Increase in external quantum efficiency of encapsulated silicon solar cells from a luminescent down‐shifting layer

This paper reports the external quantum efficiency (EQE) of encapsulated screen‐printed crystalline silicon solar cells, where the encapsulation includes a layer of luminescent down‐shifting (LDS) molecules. At wavelengths less than 400 nm, the inclusion of the LDS molecules increases the EQE from near zero to, at most, 40%. The increase in EQE corresponds to a rise in short‐circuit current density of 0·37 ± 0·13 mA/cm² under the AM1‐5g spectrum. Copyright © 2008 John Wiley & Sons, Ltd.     

Graphene Intermediate Layer in Tandem Organic Photovoltaic Cells

One strategy to harvest wide spectral solar energy is to stack different bandgap materials together in a tandem solar cell. Here, it is demonstrated that CVD grown graphene film can be employed as intermediate layer (IML) in tandem solar cells. Using MoO₃‐modified graphene IML, a high open circuit voltage (V_oc) of 1 V and a high short‐circuit current density (J_sc) of 11.6 mA cm^‐2 could be obtained in series and parallel connection, respectively, in contrast to a V_oc of 0.58 V and J_sc of 7.6 mA cm^‐2 in single PV cell. The value of V_oc (J_sc) in the tandem cell is very close to the sum of V_oc (J_sc) attained from two single subcells in series (parallel), which confirms good ohmic contact at the photoactive layer/MoO₃‐modified graphene interface. Work function engineering of the graphene IML with metal oxide is essential to ensure good charge collection from both subcells.     

p‐CuO/n‐Si heterojunction solar cells with high open circuit voltage and photocurrent through interfacial engineering

Heterojunction solar cells of p‐type cupric oxide (CuO) and n‐type silicon (Si), p‐CuO/n‐Si, have been fabricated using conventional sputter and rapid thermal annealing techniques. Photovoltaic properties with an open‐circuit voltage (V_oc) of 380 mV, short circuit current (J_sc) of 1.2 mA/cm², and a photocurrent of 2.9 mA/cm² were observed for the solar cell annealed at 300 °C for 1 min. When the annealing duration was increased, the photocurrent increased, but the V_oc was found to reduce because of the degradation of interface quality. An improvement in the V_oc resulting to a record value of 509 mV and J_sc of 4 mA/cm² with a high photocurrent of ~12 mA/cm² was achieved through interface engineering and controlling the phase transformation of CuO film. X‐ray diffraction, X‐ray photoelectron spectroscopy, and high‐resolution transmission electron microscopy analysis have been used to investigate the interface properties and crystal quality of sputter‐deposited CuO thin film. The improvement in V_oc is mainly due to the enhancement of crystal quality of CuO thin film and interface properties between p‐CuO and n‐Si substrate. The enhancement of photocurrent is found to be due to the reduction of carrier recombination rate as revealed by transient photovoltage spectroscopy analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Potential gain in photocurrent generation for Cu(In,Ga)Se2 solar cells by using In2O3 as a transparent conductive oxide layer

This study highlights the potential of atomic layer deposited In₂O₃ as a highly transparent and conductive oxide (TCO) layer in Cu(In,Ga)Se₂ (CIGSe) solar cells. It is shown that the efficiency of solar cells which use Zn‐Sn‐O (ZTO) as an alternative buffer layer can be increased by employing In₂O₃ as a TCO because of a reduction of the parasitic absorption in the window layer structure, resulting in 1.7 mA/cm² gain in short circuit current density (J_sc). In contrast, a degradation of device properties is observed if the In₂O₃ TCO is combined with the conventional CdS buffer layer. The estimated improvement for large‐scale modules is discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Preparation and characterization of pulsed laser deposited CdS/CdSe bi-layer films for CdTe solar cell application

CdS/CdSe bi-layer film was prepared by pulsed laser deposition with different substrate temperatures as an improved window layer for CdTe solar cells. The total thickness of each CdS/CdSe bi-layer film was about 70 nm, which could contribute to comparatively high transmittance of photons and, therefore, improving the photocurrent. Substrate temperature influenced the properties of the CdS/CdSe bi-layer films and the study showed that the bi-layer film prepared at 400 °C achieved the best optical transmittance and crystallinity. The crystal structure and optical transmittance of CdS/CdSe/CdTe stack before and after CdCl₂ annealing treatment were investigated by utilizing X-ray diffraction and UV/Vis spectrophotometer, respectively. It showed that further CdCl₂ annealing treatment improved the inter-diffusion of Se into CdTe, facilitating the formation of a CdTe_1−xSe_x alloy in the absorber layer. Comparing with CdTe, the alloy actually showed a smaller band gap which produced an obvious red shift of the absorption edge in long wavelength region. CdSe window layer was consumed by the inter-diffusion, while enhanced the short wavelength response in the range of 300–500 nm. The device based on CdS/CdSe window layer realized a J_SC enhancement due to the improved collection within both short and long wavelength regions accompany with a V_OC enhancement when compared to CdS/CdTe solar cell. The CdTe cell with CdS/CdSe bi-layer window deposited at 200 °C showed an efficiency of 13.47% with V_OC of 791 mV and J_SC of 27.40 mA/cm².     

Impact of silicon quantum dot super lattice and quantum well structure as intermediate layer on p‐i‐n silicon solar cells

The photovoltaic effect of the silicon (Si)/silicon carbide (SiC) quantum dot super lattice (QDSL) and multi‐quantum well (QW) strucutres is presented based on numerical simulation and experimental studies. The QDSL and QW structures act as an intermediate layer in a p‐i‐n Si solar cell. The QDSL consists of a stack of four 4‐nm Si nano disks and 2‐nm SiC barrier layers embedded in a SiC matrix fabricated with a top‐down etching process. The Si nano disks were observed with bright field‐scanning transmission electron microscopy. The simulation results based on the 3D finite element method confirmed that the quantum effect on the band structure for the QDSL and QW structures was different and had different effects on solar cell operation. The effect of vertical wave‐function coupling to form a miniband in the QDSL was observed based on the solar‐cell performance, showing a dramatic photovoltaic response in generating a high photocurrent density J_sc of 29.24 mA/cm², open circuit voltage V_oc of 0.51 V, fill factor FF of 0.74, and efficiency η of 11.07% with respect to a i‐QW solar cell with J_sc of 25.27 mA/cm², V_oc of 0.49 V, FF of 0.69, and η of 8.61% and an i‐Si solar cell with J_sc of 27.63 mA/cm², V_oc of 0.55 V, FF of 0.61, and η of 10.00%. A wide range of photo‐carrier transports by the QD arrays in the QDSL solar cell is possible in the internal quantum efficiency spectra with respect to the internal quantum efficiency of the i‐QW solar cell. Copyright © 2015 John Wiley & Sons, Ltd.     

Narrow‐bandgap CuIn3Te5 thin‐film solar cells

We propose CuIn₃Te₅ as a ternary semiconductor material for narrow‐bandgap thin‐film solar cells. Well‐developed CuIn₃Te₅ grains were obtained at a substrate temperature of 250 °C by single‐step co‐evaporation. The best solar cell that was fabricated using 4·0‐µm‐thick CuIn₃Te₅ layers grown at 250 °C yielded a total area efficiency of 6·92% (V_oc = 407 mV, J_sc = 33·1 mA/cm², and FF = 0·514). To clarify the loss in the device performance, the cell was compared with a standard CuInSe₂ reference cell. A band diagram of the CdS/CuIn₃Te₅ solar cell was also presented. Copyright © 2011 John Wiley & Sons, Ltd.     

Highly‐efficient Cd‐free CuInS2 thin‐film solar cells and mini‐modules with Zn(S,O) buffer layers prepared by an alternative chemical bath process

Recent progress in fabricating Cd‐ and Se‐free wide‐gap chalcopyrite thin‐film solar devices with Zn(S,O) buffer layers prepared by an alternative chemical bath process (CBD) using thiourea as complexing agent is discussed. Zn(S,O) has a larger band gap (E_g = 3·6–3·8 eV) than the conventional buffer material CdS (E_g = 2·4 eV) currently used in chalcopyrite‐based thin films solar cells. Thus, Zn(S,O) is a potential alternative buffer material, which already results in Cd‐free solar cell devices with increased spectral response in the blue wavelength region if low‐gap chalcopyrites are used. Suitable conditions for reproducible deposition of good‐quality Zn(S,O) thin films on wide‐gap CuInS₂ (‘CIS’) absorbers have been identified for an alternative, low‐temperature chemical route. The thickness of the different Zn(S,O) buffers and the coverage of the CIS absorber by those layers as well as their surface composition were controlled by scanning electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray excited Auger electron spectroscopy. The minimum thickness required for a complete coverage of the rough CIS absorber by a Zn(S,O) layer deposited by this CBD process was estimated to ∼15 nm. The high transparency of this Zn(S,O) buffer layer in the short‐wavelength region leads to an increase of ∼1 mA/cm² in the short‐circuit current density of corresponding CIS‐based solar cells. Active area efficiencies exceeding 11·0% (total area: 10·4%) have been achieved for the first time, with an open circuit voltage of 700·4 mV, a fill factor of 65·8% and a short‐circuit current density of 24·5 mA/cm² (total area: 22·5 mA/cm²). These results are comparable to the performance of CdS buffered reference cells. First integrated series interconnected mini‐modules on 5 × 5 cm² substrates have been prepared and already reach an efficiency (active area: 17·2 cm²) of above 8%. Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

>16% thin‐film epitaxial silicon solar cells on 70‐cm2 area with 30‐µm active layer,porous silicon back reflector,and Cu‐based top‐contact metallization

We demonstrate the use of a copper‐based metallization scheme for the specific application of thin‐film epitaxial silicon wafer equivalent (EpiWE) solar cells with rear chemical vapor deposition emitter and conventional POCl₃ emitter. Thin‐film epitaxial silicon wafer equivalent cells are consisting of high‐quality epitaxial active layer of only 30 µm, beneath which a highly reflective porous silicon multilayer stack is embedded. By combining Cu‐plating metallization and narrow finger lines with an epitaxial cell architecture including the porous silicon reflector, a J_sc exceeding 32 mA/cm² was achieved. We report on reproducible cell efficiencies of >16% on >70‐cm² cells with rear epitaxial chemical vapor deposition emitters and Cu contacts. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluation of Zn?Sn?O buffer layers for CuIn0.5Ga0.5Se2 solar cells

Thin Zn Sn O films are evaluated as new buffer layer material for Cu(In,Ga)Se₂‐based solar cell devices. A maximum conversion efficiency of 13.8% (V_oc = 691 mV, J_sc(QE) = 27.9 mA/cm², and FF = 71.6%) is reached for a solar cell using the Zn Sn O buffer layer which is comparable to the efficiency of 13.5% (V_oc = 706 mV, J_sc(QE) = 26.3 mA/cm², and FF = 72.9%) for a cell using the standard reference CdS buffer layer. The open circuit voltage (V_oc) and the fill factor (FF) are found to increase with increasing tin content until an optimum in both parameters is reached for Sn/(Zn + Sn) values around 0.3–0.4. Copyright © 2010 John Wiley & Sons, Ltd.     

Efficiency (>15%) for thin‐film epitaxial silicon solar cells on 70 cm2 area offspec silicon substrate using porous silicon segmented mirrors

We report on the beneficial use of embedded segmented porous silicon broad‐band optical reflectors for thin‐film epitaxial silicon solar cells. These reflectors are formed by gradual increase of the spatial period between the layer segments, allowing for an enhanced absorption of low energy photons in the epitaxial layer. By combining these reflectors with well‐established solar cell processing by photolithography, a conversion efficiency of 15·2% was reached on 73 cm² area, highly doped offspec multicrystalline silicon substrates. The corresponding photogenerated current densities (J_sc) were well above 31 mA/cm² for an active layer of only 20 µm. Copyright © 2010 John Wiley & Sons, Ltd.     

Flexible CdTe solar cells on polymer films

Lightweight and flexible CdTe/CdS solar cells on polyimide films have been developed in a ‘superstrate configuration’ where the light is absorbed in CdTe after passing through the polyimide substrate. The average optical transmission of the approximately 10‐μm‐thin spin‐coated polyimide substrate layer is more than ∼75% for wavelengths above 550 nm. RF magnetron sputtering was used to grow transparent conducting ZnO:Al layers on polyimide films. CdTe/CdS layers were grown by evaporation of compounds, and a CdCl₂ annealing treatment was applied for the recrystallization and junction activation. Solar cells of 8·6% efficiency with V_oc = 763 mV, I_sc = 20·3 mA/cm² and FF = 55·7% were obtained. Copyright © 2001 John Wiley & Sons, Ltd.     

Improving spectral response of monocrystalline silicon photovoltaic modules using high efficient luminescent down‐shifting Eu3+ complexes

One way to improve the spectral response of solar cells in the ultraviolet (UV) region is to convert high energy photons into lower energy ones via luminescent down‐shifting (LDS) technique. Eu³⁺ complexes are excellent LDS species because of their high luminescence quantum efficiency and large Stokes‐shift. In this paper, we aim to optimize the LDS property of Eu³⁺ complexes for monocrystalline silicon (c‐Si) photovoltaic (PV) modules by chemical modification of the UV absorbing antenna ligands. Our results show that the LDS performances of Eu³⁺ complexes are strongly dependent on their absorption and emission properties. By carefully modifying the absorption and emission features, the LDS performances of Eu³⁺ complexes can be significantly improved. The spectroscopic features of the Eu³⁺ complex with a bispinene‐containing bipyridyl ligand match well with the requirement of ideal LDS species for the c‐Si PV module. Simple coating of polyvinyl acetate film doped with this complex onto the surface of c‐Si PV module leads to increase of the external quantum efficiency in the UV region and enhancement of the PV module efficiency η (from 16.05% to 16.37%). Copyright © 2012 John Wiley & Sons, Ltd.     

Light‐trapping properties of patch textures created using laser assisted texturing

For crystalline silicon solar cells, the efficient collection of light at wavelengths in the infrared is a challenge because of long absorption lengths. Especially for thinner wafers, an efficient light‐trapping scheme, such as the patch texture, is required for high short‐circuit current densities. We have measured the light‐trapping properties of patch textures produced by laser assisted texturing (LAST) on polished ⟨100⟩silicon wafers, and compared them with ray‐tracing simulations. Single‐sided random pyramid textures are created for comparison. Excellent agreement between simulations and measurements is achieved by employing diffuse scattering with a narrow angular distribution in the simulations, confirming the successful implementation of the process. We use our optical measurements of the textures for simulations of textures with rear reflectors, where we also investigate the influence on light‐trapping properties when varying geometry and reflectance properties. The results from the optical simulations are imported into the solar cell simulation program PC1D. For a 50 μm‐thick solar cell, we simulate an improvement in J_sc of up to 0.4 mA/cm² when going from single‐sided random pyramid textures to patch textures, even when the performance of the texture is limited by process inaccuracies. Removing the physical inaccuracies of the laser system, the potential gain in J_sc on a 50 μm‐thick cell with a patch texture covering the complete wafer surface is 0.8 mA/cm². We therefore conclude that the LAST method for creating patch textures is suitable to achieve an improved J_sc in thin monocrystalline silicon solar cells. Copyright © 2013 John Wiley & Sons, Ltd.