Improved Carrier Collection Research Articles

Optimization of structural and growth parameters of metamorphic InGaAs photovoltaic converters grown by MOCVD

Metamorphic Ga0.76In0.24As heterostructures for photovoltaic converters are grown by the MOCVD (metal–organic chemical vapor deposition) technique. It is found that, due to the valence-band offset at the p-In0.24Al0.76As/p-In0.24Ga0.76As (wide-gap window/emitter) heterointerface, a potential barrier for holes arises as a result of a low carrier concentration in the wide-gap material. The use of an InAlGaAs solid solution with an Al content lower than 40% makes it possible to raise the hole concentration in the widegap window up ~9 × 1018 cm–3 and completely remove the potential barrier, thereby reducing the series resistance of the device. The parameters of an GaInAs metamorphic buffer layer with a stepwise In content profile are calculated and its epitaxial growth conditions are optimized, which improves carrier collection from the n-GaInAs base region and provides a quantum efficiency of 83% at a wavelength of 1064 nm. Optimization of the metamorphic heterostructure of the photovoltaic converter results in that its conversion efficiency for laser light with a wavelength of 1064 nm is 38.5%.

Impact of compositional grading and overall Cu deficiency on the near-infrared response in Cu(In, Ga)Se2 solar cells

Highly efficient thin film solar cells based on co-evaporated Cu(In,Ga)Se2 (CIGS) absorbers are typically grown with a [Ga]/([Ga] + [In]) (GGI) gradient across the thickness and a Cu-poor composition. Upon increasing the Cu content towards the CIGS stoichiometry, lower defect density is expected, which should lead to increased absorption in the near-infrared (NIR), diffusion length and carrier collection. Further, optimization of the GGI grading is expected to increase the NIR response. In this contribution [Cu]/([In] + [Ga]) (CGI) values are increased by shortening the deposition stage after the first stoichiometric point. In order to obtain comparable Ga contents at the interface for proper band alignment, the front GGI gradings were actively modified. With a relative CGI increase of 7%, we observe an increased photocurrent, originating from an improved NIR external quantum efficiency response. By characterizing the modified absorber properties by reflection-transmission spectroscopy, we attribute the observed behavior to changes in the optical properties rather than to improved carrier collection. Cu-dependent modifications of the NIR-absorption coefficients are likely to be responsible for the variations in the optical properties, which is supported by device simulations. Adequate re-adjustments of the co-evaporation process and of the alkali-fluorides post-deposition treatments allow maintaining Voc and FF values, yielding an overall increase of efficiency as compared to a reference baseline. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.

Thin GaAsSb capping layers for improved performance of InAs/GaAs quantum dot solar cells

This work reports on the benefits from using thin GaAsSb capping layers (CLs) on InAs/GaAs quantum dot (QD) solar cells. The application of such CLs allows the tunability of the QD ground state, switching the QD-CL band alignment from type I to type II for high Sb contents and extending the photoresponse beyond 1.5µm. Two different structures with ~10% and ~20% Sb contents in the CL (type-I and type-II band alignments, respectively) are explored, leading to efficiency improvements over a reference InAs/GaAs QD solar cell of 20% and 10%, respectively. In general, a significant increase in short-circuit current density (Jsc) is observed, partially due to the extended photocurrent spectrum and the additional contribution of the CL itself. Particularly, for a moderate Sb content, an improved carrier collection efficiency is also found to be a main reason for the Jsc increase. Calculations from an 8×8 k·p method suggest the attribution of such an improvement to longer carrier lifetimes in the wetting layer-CL structure due to the transition to a type-II band alignment. Open-circuit voltages (Voc) exceeding that of a reference QD solar cell are demonstrated under light concentration using GaAsSb CLs, which proves that the Voc is not limited by the low bandgap CLs. Moreover, the highest value is obtained for the high Sb content type-II structure, despite the higher accumulation of strain and the lower effective bandgap. Indeed, the faster Voc increase with light power found in the latter case leads to an Voc even larger than the effective bandgap.

Interstitial light-trapping design for multi-junction solar cells

We present a light-trapping design capable of significantly enhancing the photon absorption in any subcell of a multi-junction solar cell. The design works by coupling incident light into waveguide modes in one of the subcells via a diffraction grating, and preventing these modes from leaking into lower subcells via a low-index layer and a distributed Bragg reflector, which together form an omnidirectional mirror. This allows the thickness of the target subcell to be reduced without compromising photon absorption, which improves carrier collection, and therefore photocurrent. The paper focuses on using the composite structure to improve the radiation hardness of a InGaP/Ga(In)As/Ge space solar cell. In this context, it is shown via simulation that the Ga(In)As middle-cell thickness can be reduced from 3500 to 700nm, whilst maintaining strong photon absorption, and that this leads to a significantly improved end-of-life photocurrent in the Ga(In)As middle cell. However, the design can in general be applied to a wide range of multi-junction solar cell types. We discuss the principles of operation of the design, as well as possible methods of its fabrication and integration into multi-junction solar cells.

Three-dimensional self-branching anatase TiO2 nanorods with the improved carrier collection for SrTiO3-based perovskite solar cells

The organic–inorganic perovskite solar cells based on ternary oxide SrTiO3 shows a higher Voc, attributed to its slightly higher conduction band edge and better morphology of absorber material. However, its less efficient carrier collection and limited overall interfacial areas between the absorber material and the electron-transport layer (ETL), dramatically reducing the Jsc. Here, By adjusting the concentrations of the Ti(OBu)4, we successfully prepared the three-dimensional (3D) self-branching anatase TiO2 nanorod/SrTiO3 nanocomposites, and slightly tuned the particle size of SrTiO3. With the incorporation of the three-dimensional (3D) self-branching anatase TiO2 nanorod, the Jsc of the device based on SrTiO3 was highly boosted. The best performing solar cell we obtained exhibited a PCE of 9.99% with a Jsc of 19.48 mA/cm2. The excellent performance could be ascribed to the improvement of charge carrier collection of SrTiO3, better surface coverage and crystallinity of CH3NH3PbI3, and enhanced light scattering ability caused by 3D self-branching anatase TiO2 nanorods.

Germanium Alloyed Kesterite Solar Cells with Low Voltage Deficits

It has previously been shown that substitution of germanium for tin in Cu2ZnSn(S,Se)4 provides a means to change the band gap. Here, we show that Ge substitution can also be used to improve carrier collection and decrease the open-circuit voltage deficit that has hindered kesterites. Using a simple molecular ink, we spray coat continuous composition gradients of Cu2Zn(Sn,Ge)(S,Se)4 spanning 0–90% Ge/(Ge + Sn). By mapping the absolute intensity photoluminescence and making devices from these gradients, we are able to clearly resolve changes in material properties and device performance with composition. We find that Ge can be used to increase the band gap as high as 1.3 eV (50% Ge/(Ge + Sn)) without any loss in optoelectronic material quality, but beyond 50% Ge/(Ge + Sn), the device efficiency decreases rapidly. We show evidence that the degradation results from both a deep defect located about 0.8 eV above the valence band and unfavorable band alignment. As the band gap increases, the defect moves toward ...

Concurrent improvement in optical and electrical characteristics by using inverted pyramidal array structures toward efficient Si heterojunction solar cells

The Si heterojunction (SHJ) solar cell is presently the most popular design in the crystalline Si (c-Si) photovoltaics due to the high open-circuit voltages (VOC). Photon management by surface structuring techniques to control the light entering the devices is critical for boosting cell efficiency although it usually comes with the VOC loss caused by severe surface recombination. For the first time, the periodic inverted pyramid (IP) structure fabricated by photolithography and anisotropic etching processes was employed for SHJ solar cells, demonstrating concurrent improvement in optical and electrical characteristics (i.e., short-circuit current density (JSC) and VOC). Periodic IP structures show superior light-harvesting properties as most of the incident rays bounce three times on the walls of the IPs but only twice between conventional random upright pyramids (UPs). The high minority carrier lifetime of the IP structures after a-Si:H passivation results in an enhanced VOC by 28mV, showing improved carrier collection efficiency due to the superior passivation of the IP structure over the random UP structures. The superior antireflective (AR) ability and passivation results demonstrate that the IP structure has the potential to replace conventional UP structures to further boost the efficiency in solar cell applications.

Experimental demonstration and modeling of the internal light scattering profile within solar cells due to random dielectric scatterers

Many photovoltaic technologies are shifting toward thin-film devices to simultaneously reduce costs and improve carrier collection efficiencies; however, the need for nearly complete light absorption within the semiconductor to achieve large short-circuit currents constrains this thickness reduction. Light trapping strategies can be employed to increase absorption in thinner devices. Random scattering coatings offer a simple, cost-effective way to increase solar cell absorption without the drawback of increased surface recombination or reduced bandwidth that occurs when using surface texturing or gratings. However, coatings that show excellent performance as scatterers in free space generally do not enhance device absorption as much as an ideal Lambertian scatterer. Here, we present an experimental technique and theoretical model that accurately describes the absorption improvement that is achievable with coatings based on random ensembles of dielectric scatterers. We find that the ideal Lambertian model substantially overestimates the experimental scattering results, but significant path length enhancements are still achievable. The experimental techniques presented here should enable the testing of various optical models that attempt to surpass the ray optics light trapping limit, which have in many cases been hindered by the experimental difficulty of coupling the incident light into the optical modes of the absorber.

Annealing Effects in Low Temperature Amorphous Silicon Flexible Solar Cells

We report the effects of prolonged post-deposition annealing on the performance of amorphous silicon (a–Si:H) solar cells and single layers, that are fabricated at low temperature of 120 ̊C on flexible PET and glass substrates. These low temperature solar cells show a significant improvement in performance upon post-deposition annealing (up to two hours) in all parameters of the current-voltage (JV) curves of the solar cell, resulting in an efficiency increase up to 34% (relative). Comparison of external quantum efficiencies of p- and n-side illuminated cells, as well as varied absorber layer thicknesses and reverse bias voltage, suggest that the collection of minority carriers are mainly improved upon annealing. Both the increase of the built-in field in the solar cell and improvement in the electronic properties of the absorber layer were found to contribute to an improved carrier collection and solar cell performance.

SiNx layers on nanostructured Si solar cells: Effective for optical absorption and carrier collection

We compared nanopatterned Si solar cells with and without SiNx layers. The SiNx layer coating significantly improved the internal quantum efficiency of the nanopatterned cells at long wavelengths as well as short wavelengths, whereas the surface passivation helped carrier collection of flat cells mainly at short wavelengths. The surface nanostructured array enhanced the optical absorption and also concentrated incoming light near the surface in broad wavelength range. Resulting high density of the photo-excited carriers near the surface could lead to significant recombination loss and the SiNx layer played a crucial role in the improved carrier collection of the nanostructured solar cells.

Enhanced power conversion efficiency in InGaN-based solar cells via graded composition multiple quantum wells.

This work demonstrates the enhanced power conversion efficiency (PCE) in InGaN/GaN multiple quantum well (MQWs) solar cells with gradually decreasing indium composition in quantum wells (GQWs) toward p-GaN as absorber. The GQW can improve the fill factor from 42% to 62% and enhance the short current density from 0.8 mA/cm2 to 0.92 mA/cm2, as compares to the typical MQW solar cells. As a result, the PCE is boosted from 0.63% to 1.11% under AM1.5G illumination. Based on simulation and experimental results, the enhanced PCE can be attributed to the improved carrier collection in GQW caused by the reduction of potential barriers and piezoelectric polarization induced fields near the p-GaN layer. The presented concept paves a way toward highly efficient InGaN-based solar cells and other GaN-related MQW devices.

A special section on Solar cells, light emitting diodes and photodetectors

Solar cells, light emitting diodes and photodetectors are the three typical optoelectronic devices that play an important role in the welfare of human beings. Solar cell is a diode that absorbs solar irradiation and converts the incident photons into electron-hole pairs and then circulates and powers the external load. Light emitting diodes are also diodes that convert the injected electrons and holes, radiatively recombine within the active layer and then emit photons to light the world. Photodetector, normally either a diode or a resistor, also convert incident photons to output current, in analogy to solar cells. However, they distinct from solar cells in that photodetectors usually are biased during operation and their function is to detect light instead of utilize light for energy. All these three type of devices are actively and intensively studied in the academia, achieving better overall performance with lower cost being the ever-lasting goals. Consequently, new materials, new device configuration and new systematic integration are continuously proposed and investigated like bamboo shoots after a spring rain. In this section, two review papers on the emerging photovoltaics, colloidal quantum dot (CQD) and copper zinc tin sulfoselenide (CZTSSe) solar cells, are presented. CQD photovoltaics utilize semiconductors in a nanoscale dimension as the active absorber layer, which possess the advantage of tunable band gap into the infrared and low cost solution processing. In the review paper presented by Prof. XihuaWang from University of Alberta, the research progress in CQD photovoltaics from 2011, a best 5.1% efficiency, to 2015, a record efficiency of 9.9%, was thoroughly reviewed with special focus on the theoretical and experimental works that improve carrier collection efficiency and minimize recombination loss within CQD ensemble layer. CZTSSe is a new emerging absorber material for thin film photovoltaics in the hope that the expensive gradient indium in the well-established copper indium gallium diselenide (CIGS) solar cells could be replaced by inexpensive and earth abundant zinc and tin. Various strategies including vacuum based and solution based process were adopted to produce efficient CZTSSe solar cells. In the review paper written by Dr. Jie Zhong and Prof. Jiang Tang from Wuhan National Laboratory for Optoelectronics (WNLO), an environmentally friendly solution process using benign solvents to produce CZTSSe inks is reviewed. Key steps associated with this process including ink preparation and stabilization, film fabrication, device integration and performance are carefully analyzed and summarized. In addition to the review paper, a research paper employing poly(dimethyl diallylammonium chloride) (PDDA) layers to reduce the work function of ITO substrate is also presented by Prof. Yan Shen from WNLO, reporting increased short circuit current and enhanced device performance after their modification. In the other two topics, light emitting diode and photodetector, two research paper are presented, each focusing on one topic. Organic light emitting diodes consume less power and have larger fields of view than conventional LCDs and have shown many competitive applications in the market. In the paper presented by Prof. Zujin Zhao from Hangzhou Normal University and Prof. Benzhong Tang from The Hong Kong University of Science & Technology, aggregation induced emission from melted tetraphenylethene (TPE) and 1,2,4,5-tetraphenyl-1H-imidazole (TPI) units mixture are explored for blue fluorophores and then successfully integrated into light emitting diodes demonstrating electroluminescence at 467 nm (CIE = 0.17, 0.22) with good efficiencies of 3.17 cd$A and external

Recent progress in colloidal quantum dot photovoltaics

The development of photovoltaic devices, solar cells, plays a key role in renewable energy sources. Semiconductor colloidal quantum dots (CQDs), including lead chacolgenide CQDs that have tunable electronic bandgaps from infrared to visible, serve as good candidates to harvest the broad spectrum of sunlight. CQDs can be processed from solution, allowing them to be deposited in a roll-to-roll printing process compatible with low-cost fabrication of large area solar panels. Enhanced multiexciton generation process in CQD, compared with bulk semiconductors, enables the potential of exceeding Shockley-Queisser limit in CQD photovoltaics. For these advantages, CQDs photovoltaics attract great attention in academics, and extensive research works accelerate the development of CQD based solar cells. The record efficiency of CQD solar cells increased from 5.1% in 2011 to 9.9% in 2015. The improvement relies on optimized material processing, device architecture and various efforts to improve carrier collection efficiency. In this review, we have summarized the progress of CQD photovoltaics in year 2012 and after. Here we focused on the theoretical and experimental works that improve the understanding of the device physics in CQD solar cells, which may guide the development of CQD photovoltaics within the research community.

Use of Sub-bandgap Illumination to Improve Radiation Detector Resolution of CdZnTe

The performance of Cd1−x Zn x Te (CZT) materials for room-temperature gamma/x-ray radiation detection continues to improve in terms of material quality and detector design. In our prior publications, we investigated the use of multiple wavelengths of light (in the visible and infrared) to target charge carriers at various trap energies and physical positions throughout crystals. Light exposure significantly alters the charge mobility and improves carrier collection at the anode contact. This study presents an investigation of material performance as a radiation detector during such illumination. The decrease in charge trapping and increase in charge collection due to a higher probability of free electron release from traps contributed to an increase in the resolution-based performance of the detector through controlled illumination. We investigated the performance improvement of CZT crystals with previously known levels of intrinsic defects and secondary phases, at various voltages, light-emitting diode (LED) light wavelengths, and shaping times. Although our setup was clearly not optimized for radiation detector performance, it demonstrated substantial resolution improvements (based on full-width at half-maximum using 662-keV gamma rays from 137Cs upon illumination with 950-nm light) of 16% to 38% in comparison with unilluminated CZT under similar conditions. This manuscript includes discussion of the electrooptic behavior and its effect on performance. Additional testing and fabrication of a detector that incorporates such LED light optimization could lead to improved performance with existing detector-grade materials.

Characteristics of Carrier Collection of Nanopillar-Patterned Silicon Solar Cells.

Nanopillar-patterned Si solar cells were investigated. Ag nanoparticles were coated on a polished Si substrate as an etching mask. Reactive ion etching caused Si nanopillars to replicate in a reverse fashion on the Ag nanoparticles over a large area. The nanopillar structures efficiently reduced the light reflection on the surface and effectively drove the incident light into a Si absorber. This induced a significant enhancement of the photogenerated-current with an improved solar cell efficiency of 16.07%. The Si nanopillar-patterned solar cells showed improved carrier collection for long wavelengths; however, the surface-defect induced recombination degraded the quantum efficiency at short wavelengths. We suggest that the reduction of recombination loss should be considered for efficient nanostructure solar cells.

Enhancing Photovoltaic Performance Using an All‐Conjugated Random Copolymer to Tailor Bulk and Interfacial Morphology of the P3HT:ICBA Active Layer

Bulk heterojunction (BHJ) solar cells are fabricated using active material blends of poly(3‐hexylthiophene) (P3HT) donor, indene‐C60 bisadduct (ICBA) acceptor, and an all‐conjugated random copolymer (RCP) additive. By optimizing RCP loading, power conversion efficiencies (PCEs) up to 20% higher than those of a binary P3HT:ICBA mixture are achieved. The improved device characteristics are rationalized in terms of the differences between the photoactive thin film morphologies. Energy‐filtered transmission electron micro­scopy reveals that incorporation of the RCP improves the degree of structural order of the BHJ fibrillar network and increases the extent of microphase separation between P3HT and ICBA. Additionally, a combination of atomic force microscopy and X‐ray photoelectron spectroscopy analysis indicates segregation of the RCP at the free interface, leading to a shift in the surface potentials measured by Kelvin probe force microscopy. These changes, both in the bulk morphology and in the interfacial composition/energetics, are correlated to improved carrier collection efficiency due to a reduction of non‐geminate recombination, which is measured by charge extraction of photo­generated carriers by linearly increasing voltage.

Doping of polycrystalline CdTe for high-efficiency solar cells on flexible metal foil

Roll-to-roll manufacturing of CdTe solar cells on flexible metal foil substrates is one of the most attractive options for low-cost photovoltaic module production. However, various efforts to grow CdTe solar cells on metal foil have resulted in low efficiencies. This is caused by the fact that the conventional device structure must be inverted, which imposes severe restrictions on device processing and consequently limits the electronic quality of the CdTe layer. Here we introduce an innovative concept for the controlled doping of the CdTe layer in the inverted device structure by means of evaporation of sub-monolayer amounts of Cu and subsequent annealing, which enables breakthrough efficiencies up to 13.6%. For the first time, CdTe solar cells on metal foil exceed the 10% efficiency threshold for industrialization. The controlled doping of CdTe with Cu leads to increased hole density, enhanced carrier lifetime and improved carrier collection in the solar cell. Our results offer new research directions for solving persistent challenges of CdTe photovoltaics.

Electrochemical Nanostructuring of n-GaAs Photoelectrodes

Methods to simultaneously optimize carrier collection and light in-coupling in semiconductors are important for developing low-cost, high-efficiency photovoltaics and photoelectrodes. We anodically etched nanostructures into planar (100) n-GaAs wafers with different bulk minority carrier diffusion lengths L(D). The structures were varied by changing the anodization parameters. A ferrocene/ferrocenium electrolyte provided a conformal rectifying contact to the anodized n-GaAs and enabled the measurement of carrier generation and collection as a function of nanostructure geometry and L(D). Internal quantum efficiency Φ(int) of photoelectrodes varied with nanostructure geometry and L(D). External quantum efficiency Φ(ext) also depended on the reflectance of the nanostructured GaAs-electrolyte interface. Reflectance was minimized using anodization current densities of 100-150 mA cm(-2), which etched subwavelength trigonal prismatic nanostructures ~400 nm in width at their base. For Si-doped n-GaAs with L(D) = 170 nm, peak Φ(ext) of ~75% and Φ(int) of ~85% was achieved using J(anod) = 150 mA cm(-2). The control of both surface nanostructure (to minimize reflection) and pore depth and spacing (to optimize 3D carrier collection) via two-step anodization yielded photoelectrodes with peak Φ(ext) of ~85% and peak Φ(int) of ~95% for Te-doped n-GaAs with a bulk L(D) of only 420 nm. The measured short-circuit current densities for the nanostructured photoelectrodes were up to 2.5 times that of planar controls, demonstrating that appropriate nanostructuring significantly improves carrier collection even for direct bandgap materials with large absorption coefficients like GaAs.

Electric field engineering using quantum-size-effect-tuned heterojunctions

A quantum junction solar cell architecture was recently reported that employs colloidal quantum dots (CQDs) on each side of the p-n junction. This architecture extends the range of design opportunities for CQD photovoltaics, since the bandgap can be tuned across the light-absorbing semiconductor layer via control over CQD size, employing solution-processed, room-temperature fabricated materials. We exploit this feature by designing and demonstrating a field-enhanced heterojunction architecture. We optimize the electric field profile within the solar cell through bandgap engineering, thereby improving carrier collection and achieving an increased open circuit voltage, resulting in a 12% improvement in power conversion efficiency.

Graded Doping for Enhanced Colloidal Quantum Dot Photovoltaics

A novel approach to improving all-inorganic colloidal quantum dot (CQD) homojunction solar cells by engineering the doping spatial profile to produce a doping gradient within the n-type absorber is presented. The doping gradient greatly improves carrier collection and enhances the voltages attainable by the device, leading to a 1 power point power conversion efficiency (PCE) improvement over previous inorganic CQD solar cells.