Reduced Interface Recombination Research Articles

An investigation on the relationship between open circuit voltage and grain size for CZTSSe thin film solar cells fabricated by selenization of sputtered precursors

The low open circuit voltage (Voc) of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells limits their efficiency. CZTSSe absorbers were fabricated by sputtering Cu2ZnSnS4 (CZTS) target and subsequent selenization treatment and then incorporated into solar cells. The influence of selenization temperature on the growth of the CZTSSe grains and device performance was examined. The absorber films were composed of a CZTSSe phase with high Se/(Se+S) ratios. As the selenization temperature was increased from 460 °C to 500 °C, the grains grew from the top to the bottom of the CZTSSe absorbers, and the average Voc of the CZTSSe solar cells increased from 284 mV to 371 mV. The band gaps (Eg), derived from external quantum efficiency (EQE) data, were approximately 1.11 eV. Activation energies (Ea) was extracted from temperature-dependent current density-voltage (J-V) measurements and used to evaluate the interface recombination level. The Ea increased from 0.82 eV to 0.89 eV as the selenization temperature was increased, which approached the Eg of CZTSSe. This was likely caused by reduced interface recombination because the grain boundaries decreased as the grains grew larger. An approximately linear relationship between the grain size and Voc was observed. The increase of grain size was achieved by optimizing the selenization temperature, which reduced interface recombination and resulted in an increased Voc. CZTSSe solar cells were fabricated with a maximum efficiency of 8.97%.

From sputtered metal precursors towards Cu2Zn(Sn1-x,Gex)Se4 thin film solar cells with shallow back grading

Bandgap grading is often employed in thin film solar cell absorbers for creating the back surface field that can reduce interface recombination at the back contact. Here, we investigate different pathways to obtain back graded Cu2Zn(Sn1-x,Gex)Se4 thin film solar cells based on a co-sputtered metal precursor and rapid thermal annealing route. The absorber bandgap can be precisely tuned for the whole compositional range of x = 0…1. While Ge does not accumulate towards the back in absorbers fabricated from uniform precursor, Ge-back graded absorbers can be obtained from stacked metal precursors. A linear back grading with a bandgap energy difference of up to 40 meV has been achieved. However, no significant improvement in open-circuit voltage and near-infrared response could be observed for the kesterite devices. This indicates that even steeper gradients are required to obtain an effective back surface field.

Wet Processing in State-of-the-Art Cu(In,Ga)(S,Se)<sub>2</sub> Thin Film Solar Cells

Interface quality plays a key role in solar cell applications. Interface recombination at the front and rear surfaces, which determine this quality, have significant effects on open circuit voltage and fill factor values. In this work, several surface treatments were applied on Cu(In,Ga)Se2 (CIGS) surfaces to improve the interface quality. Besides, the passivation layer implementation was investigated to reduce interface recombination between the buffer and absorber layers.

Phosphorus diffused LPCVD polysilicon passivated contacts with in-situ low pressure oxidation

As silicon photovoltaic technology advances, charge carrier losses at the contacted interfaces of the silicon absorber are coming to dominate power conversion efficiency. The so-called passivated contact, which provides selective charge-carrier extraction while simultaneously reducing interface recombination, is thus of significant interest for next-generation silicon solar cells. However, achieving both low recombination and low resistance to charge carrier extraction has proven challenging. Here, we present a passivated contact technology based on polysilicon deposited using low pressure chemical vapour deposition (LPCVD) over an ultra-thin silicon dioxide layer, which achieves an excellent surface passivation with implied open-circuit voltage of 735 mV, a recombination prefactor below 1 fA cm−2 and contact resistivity below 1 mΩ cm2.Key to this technology is the deposition of an ultra-thin silicon dioxide interlayer under high temperature and low pressure condition, performed in-situ within a single process with the polysilicon deposition. Additionally, the passivating contact structure maintains its electronic properties at temperatures of up to 900 °C and is compatible with existing industrial processes. The presented work therefore represents a significant advancement in industrially-applicable passivated contact technology.

Improving the charge separation and collection at the buffer/absorber interface by double-layered Mn-substituted CZTS

A non-ideal buffer-absorber interface due to interface recombination is one of the limitations in Cu2ZnSnS4 (CZTS) solar cells. An absorber gradient using partial cation-substitution is one possible solution to this issue by reducing interface recombination sites and improving band alignment. Partial substitution of Zn with Mn is a potentially attractive concept owing to its abundance as well as similar size and electronic properties compared to Zn. In this study, Cu2MnxZn1-xSnS4 (CMZTS) thin film were fabricated by a sol-gel spin coating technique to understand the effect of Mn on the device performance. X-ray diffraction (XRD) and Raman analysis confirm Mn substitution for Zn in the crystal lattice of Cu2ZnSnS4 (CZTS) with no secondary phases detected for x ≤ 0.4. Enhancement of short circuit current (Jsc) in Cu2Mn0.15Zn0.85SnS4 has led to improved efficiency in comparison with CZTS. Since external quantum efficiency (EQE) indicates the short wavelength collection as the main reason for the improved Jsc, we designed a double layered structure of CZTS as bottom layer and CM0.15Z0.85TS as top layer. With this structure, we obtained our best performance cell with power conversion efficiency (PCE) of 5.73% and Jsc of 17.86 mA/cm2. The improved Jsc is attributed to increased depletion width and higher activation energy of the limiting recombination mechanism as shown by capacitance-voltage and temperature dependent current-voltage measurements. Our study concludes that CM0.15Z0.85TS top layer improves the interface quality of the p-n junction and may be an alternative method to improve the performance of CZTS solar cells.

Surface and bulk effects of K in highly efficient Cu1-xKxInSe2 solar cells

To advance knowledge of the beneficial effects of K in Cu(In,Ga)(Se,S)2 (CIGS) photovoltaic (PV) absorbers, recent Cu-K-In-Se phase growth studies have been extended to PV performance. First, the effect of distributing K throughout bulk Cu1-xKxInSe2 absorbers at low K/(K+Cu) compositions (0 ≤ x ≤ 0.30) was studied. Efficiency, open-circuit voltage (VOC), and fill factor (FF) were greatly enhanced for x ~ 0.07, resulting in an officially-measured 15.0%-efficient solar cell, matching the world record CuInSe2 efficiency. The improvements were a result of reduced interface and bulk recombination, relative to CuInSe2 (x ~ 0). However, higher x compositions had reduced efficiency, short-circuit current density (JSC), and FF due to greatly increased interface recombination, relative to the x ~ 0 baseline. Next, the effect of confining K at the absorber/buffer interface at high K/(K+Cu) compositions (0.30 ≤ x ≤ 0.92) was researched. Previous work showed that these surface layer growth conditions produced CuInSe2 with a large phase fraction of KInSe2. After optimization (75 nm surface layer with x ~ 0.41), these KInSe2 surface samples exhibited increased efficiency (officially 14.9%), VOC, and FF as a result of decreased interface recombination. The KInSe2 surfaces had features similar to previous reports for KF post-deposition treatments (PDTs) used in world record CIGS solar cells—taken as indirect evidence that KInSe2 can form during these PDTs. Both the bulk and surface growth processes greatly reduced interface recombination. However, the KInSe2 surface had higher K levels near the surface, greater lifetimes, and increased inversion near the buffer interface, relative to the champion bulk Cu1-xKxInSe2 absorber. These characteristics demonstrate that K may benefit PV performance by different mechanisms at the surface and in the absorber bulk.

Sputter-Deposited Oxides for Interface Passivation of CdTe Photovoltaics

Commercial CdTe PV modules have polycrystalline thin films deposited on glass, and devices made in this format have exceeded 22% efficiency. Devices made by the authors with a magnesium zinc oxide window layer and tellurium back contact have achieved efficiency over 18%, but these cells still suffer from an open-circuit voltage far below ideal values. Oxide passivation layers made by sputter deposition have the potential to increase voltage by reducing interface recombination. CdTe devices with these passivation layers were studied with photoluminescence (PL) emission spectroscopy and time-resolved photoluminescence (TRPL) to detect an increase in minority carrier lifetime. Because these oxide materials exhibit barriers to carrier collection, micropatterning was used to expose small point contacts while still allowing interface passivation. TRPL decay lifetimes have been greatly enhanced for thin polycrystalline absorber films with interface passivation. Device performance was measured and current collection was mapped spatially by light-beam-induced current.

Sulfurization of Co-Evaporated Cu(In,Ga)Se2 as a Postdeposition Treatment

It is investigated if the performance of Cu(In,Ga)Se2 (CIGSe) solar cells produced by co-evaporation can be improved by surface sulfurization in a postdeposition treatment. The expected benefit would be the formation of a sulfur/selenium gradient resulting in reduced interface recombination and increased open-circuit voltage. In the conditions used here it was, however, found that the reaction of the CIGSe layer in a sulfur environment results in the formation of a CuInS2 (CIS) surface phase containing no or very little selenium and gallium. At the same time, a significant pile up of gallium was observed at the CIGSe/CIS boundary. This surface structure was formed for a wide range of annealing conditions investigated in this paper. Increasing the temperature or extending the time of the dwell stage had a similar effect on the material. The gallium enrichment and CIS surface layer widens the surface bandgap and therefore increases the open-circuit voltage. At the same time, the fill factor is reduced, since the interface layer acts as an electron barrier. Due to the balance of these effects, the conversion efficiency could not be improved.

Evaluation of recombination losses in thin film solar cells using an LED sun simulator − the effect of RbF post-deposition on CIGS solar cells

Distinguishing among different electrical loss mechanisms − such as interface and bulk recombination − is a common problem in thin film solar cells. In this work, we report a J–V measurement technique using different illuminating spectra to distinguish between these two recombination losses. The basic idea is to change the relative contribution of bulk recombination to the total losses of photo-generated charge carriers by generating them in different depths within the absorber layer using different spectral regions of the illuminating light. The use of modern LED sun-simulators allows an almost free design of illumination spectra at intensities close to 1 sun. The comparison of two simple J–V measurements, one recorded with illumination near the absorber's band-gap energy and one with light of higher energy, in combination with supporting measurements of the absorber properties, as well as device modeling, enables the extraction of the diffusion length and the interface recombination velocity. Using this technique, we show that in CIGS solar cells, an RbF post-deposition treatment does not only reduce interface recombination losses, as often reported, but also reduces bulk recombination in the CIGS absorber. Furthermore, we find that both cells, with and without RbF treatment, are dominantly affected by interface recombination losses.

Enhanced Efficiency of Cd-Free Cu(In,Ga)(Se,S)2 Minimodule Via (Zn,Mg)O Second Buffer Layer and Alkali Metal Post-Treatment

New record efficiencies have been achieved on Cd-free Cu(In,Ga)(Se,S)2 thin-film photovoltaic submodules prepared by a two-step sulfurization after selenization process. Aperture area efficiencies of 19.2% and 19.8% were independently confirmed on a 30 cm × 30 cm submodule (841 cm2) and on a 7 cm × 5 cm minimodule (24.2 cm2), respectively. These achievements were brought about by transferring several key techniques, especially atomic layer deposited (Zn,Mg)O second buffer layer and K treatment of the absorber surface, from the fundamental study of small-area cell development. The former technique was applied to the submodule and both techniques were implemented into the minimodule. The (Zn,Mg)O second buffer layer increases transmittance of the window layer and improves junction quality resulting in the reduced interface recombination. The K treatment, which was developed by reference to the postdeposition treatment widely used in the co-evaporation process, significantly enhances open-circuit voltage and fill factor. Several material and device characterizations performed to illuminate the effects of the K treatment showed that increased free carrier concentration and reduced carrier recombination throughout the whole absorber film contributed to the improved performance. Contrary to the conventional postdeposition treatment in the co-evaporation process, significant depletion of Cu at the absorber surface was not observed, which can be attributed to S-rich circumstances of our absorber surface. The achievement of nearly 20% efficiency on the minimodule having identical structure to the production modules ensures further performance improvements in industrial Cu(In,Ga)(Se,S)2 modules in the near future.

Reducing Interface Recombination through Mixed Nanocrystal Interlayers in PbS Quantum Dot Solar Cells.

The performance of ZnO/PbS colloidal quantum dot (CQD)-based heterojunction solar cells is hindered by charge carrier recombination at the heterojunction interface. Reducing interfacial recombination can improve charge collection and the photocurrent of the device. Here we report the use of a mixed nanocrystal (MNC) buffer layer comprising zinc oxide nanocrystals and lead sulfide quantum dots at the respective heterojunction interface. Remote trap passivation of the PbS CQDs taking place within this MNC layer reduces interfacial recombination and electron back transfer which in turn improves charge collection efficiency. Upon the addition of the MNC layer, the overall power conversion efficiency increases from 9.11 to 10.16% and Short-circuit current density (JSC) increases from 23.54 to 25.23 mA/cm2. Optoelectronic characterization of the solar cells confirms that the effects underlying device improvement are reduced trap density and improved charge collection efficiency due to the presence of the MNC buffer layer.

Review of grain interior, grain boundary, and interface effects of K in CIGS solar cells: Mechanisms for performance enhancement

Introducing K into Cu(In,Ga)(Se,S)2 (CIGS) absorbers has led to recent world record power conversion efficiencies for thin film polycrystalline solar cells. In this work, the diverse phenomena associated with K in CIGS were reviewed, and overarching mechanisms were identified. The effects of K depend on its distribution among grain interiors (GIs), grain boundaries (GBs), and interfaces. High substrate Na and low temperature favor GI K incorporation, while low Na and high temperature favor segregation of K at GBs. Depositing KInSe2 (or KIn1−yGaySe2) by co-evaporation or KF post-deposition treatment onto CIGS reduces buffer interface recombination in the final solar cells. KInSe2 decomposes in air, which makes characterization difficult and may affect performance. The mechanism for reduced interface recombination could be direct passivation, beneficial compound precursor, oxidation barrier, or favorable diffusion alteration.

Record 1.0 V open‐circuit voltage in wide band gap chalcopyrite solar cells

AbstractTandem solar cell structures require a high‐performance wide band gap absorber as top cell. A possible candidate is CuGaSe2, with a fundamental band gap of 1.7 eV. However, a significant open‐circuit voltage deficit is often reported for wide band gap chalcopyrite solar cells like CuGaSe2. In this paper, we show that the open‐circuit voltage can be drastically improved in wide band gap p‐Cu(In,Ga)Se2 and p‐CuGaSe2 devices by improving the conduction band alignment to the n‐type buffer layer. This is accomplished by using Zn1−xSnxOy, grown by atomic layer deposition, as a buffer layer. In this case, the conduction band level can be adapted to an almost perfect fit to the wide band gap Cu(In,Ga)Se2 and CuGaSe2 materials. With an improved buffer band alignment for CuGaSe2 absorbers, evaporated in a 3‐stage type process, we show devices exhibiting open‐circuit voltages up to 1017 mV, and efficiencies up to 11.9%. This is to the best of our knowledge the highest reported open‐circuit voltage and efficiency for a CuGaSe2 device. Temperature‐dependent current‐voltage measurements show that the high open‐circuit voltage is explained by reduced interface recombination, which makes it possible to separate the influence of absorber quality from interface recombination in future studies.

Optimisation of rear reflectance in ultra-thin CIGS solar cells towards >20% efficiency

In order to decrease their cost and the use of rare metal elements, thin film solar cell thicknesses are continuously reduced at the expense of their efficiency, due to a lack of absorption for long wavelengths. Optimisation of cells rear reflectance (Rb) thus becomes meaningful to provide non-absorbed light a second chance to be harvested by the active cell layer. In this sense, we present a way to keep the rear reflectance in advanced Cu(In, Ga) Se2 (CIGS) cell as high as possible while keeping in mind the progress already done regarding the rear passivation techniques. We show that introducing a stack of thin Al2O3 and aluminium between the CIGS layer and the rear molybdenum electrode increases Rb up to 92% in the long wavelength 800–1100nm range. Several other stacks, using MgF2, SiO2 or TiO2, are optimised in order to investigate the best trade-off between passivation, material consumption and performances, resulting in Rb ranging from 42% (moderate case) to 99% in the best case. Those CIGS rear interface reflectance optimisations were performed by using a standard transfer matrix method (TMM) in the long wavelength range. Seven interesting stacks are then analysed for solar cell performances using SCAPS simulation software to understand the impact of rear reflectance on short circuit current density (Jsc) and eventually on the cell efficiency (η), for ultra-thin CIGS absorber thicknesses (<1μm). Based on these results, we propose Rb optimisation to achieve Jsc>40mA/cm2 and η>20% with a 500nm-thick CIGS absorber film using CIGS-Al2O3-Mo stack, where the Al2O3 thickness can be chosen in between 104 and 139nm. This way, we can ensure good rear reflectance (Rb=65%) and reduced interface recombination while being industrially feasible with present technologies.

An easily prepared carbon quantum dots and employment for inverted organic photovoltaic devices

In this paper, the performance enhancement of organic solar cells based on PCDTBT:PC71BM and P3HT:PC60BM are demonstrated via employing carbon quantum dots (CQD) to modify the metal oxide electron transport layer. The incorporated CQD with carboxylic acid (–COOH) groups could induce self-assembled monolayer (SAM) with TiO2 buffer layer, which lowered the energy barrier for electron transfer and reduced the inherent incompatibility between the metal oxide and organic active layers. Further investigation shows that SAM of TiO2 with CQD can improve the photo-induced exciton dissociation and charge transfer, leading to a low electron accumulation in charge transport layer and a reduced interface recombination loss.

Interface passivation using ultrathin polymer–fullerene films for high-efficiency perovskite solar cells with negligible hysteresis

Reducing interface recombination boosts the Voc for perovskite solar cells.

Effect of CdS deposition temperature for thermally grown Cu(In,Ga)Se2 and two-step chalcogenized Cu(In,Ga)(S,Se)2 absorbers on solar cell performances

CdS buffer layer of varying thickness ranging from 23 to 58 nm deposited at different substrate temperature were prepared as n-type junction partner for thermally grown Cu(In,Ga)Se2 and two-step chalcogenized Cu(In,Ga)(S,Se)2 photovoltaic absorber films and the effect of deposition temperature and time on the CdS growth behavior and solar cell performance were evaluated. High deposition temperature resulted in a thicker CdS layer and more importantly lower density and shallower depth of open voids, which attributed to the improved open-circuit voltage and fill factor due to reduced interface recombination. The solar cell efficiency of thermally grown absorber saturated at about 30 nm thickness of CdS, while that of chalcogenized absorber gradually increased with CdS thickness up to 60 nm without significant loss of short-circuit current density.

Photovoltaic properties of Cu2ZnSnS4 cells fabricated using ZnSnO and ZnSnO/CdS buffer layers

To improve the photovoltaic properties of Cu2ZnSnS4 (CZTS) photovoltaic cells, we investigated the use of novel buffer layer materials. We found that Zn1−xSnxOy fabricated by atomic layer deposition functioned as an effective buffer layer. The short-circuit current density increased by 10% because of a decrease in the absorption loss in the short-wavelength region. With Zn0.70Sn0.30Oy layers, the conversion efficiency was 5.7%. To reduce interface recombination, a thin CdS layer was inserted between the ZnSnO and CZTS layers. The CZTS cells fabricated using ZnSnO/CdS double buffer layers showed a high open-circuit voltage of 0.81 V.

Improvement of kesterite solar cell performance by solution synthesized MoO3 interfacial layer

In this study, an ultra‐thin MoO3 layer synthesized by a solution‐based technique is introduced as a promising interfacial layer to improve the performance of kesterite Cu2ZnSnSe4 (CZTSe) solar cell. Solar cells with 10 nm of MoO3 between Mo rear contact and CZTSe had larger minority carrier life time and open‐circuit voltage compared to the reference solar cells. Temperature dependent current density–voltage measurement indicated that the activation energy (EA) of the main recombination is higher (∼ 837 meV) in solar cells with MoO3 layer, as compared to conventional solar cells where EA ∼ 770 meV, indicating reduced interface recombination. A best efficiency of 7.1% was achieved for a SLG/Mo/MoO3/CZTSe/CdS/TCO solar cell compared to the reference solar cell SLG/Mo/CZTSe/CdS/TCO for which 5.9% efficiency was achieved.

Reduced defect density at the CZTSSe/CdS interface by atomic layer deposition of Al2O3

The greatest challenge for improving the power conversion efficiency of Cu2ZnSn(S,Se)4 (CZTSSe)/CdS/ZnO thin film solar cells is increasing the open circuit voltage (VOC). Probable leading causes of the VOC deficit in state-of-the-art CZTSSe devices have been identified as bulk recombination, band tails, and the intertwined effects of CZTSSe/CdS band offset, interface defects, and interface recombination. In this work, we demonstrate the modification of the CZTSSe absorber/CdS buffer interface following the deposition of 1 nm-thick Al2O3 layers by atomic layer deposition (ALD) near room temperature. Capacitance-voltage profiling and quantum efficiency measurements reveal that ALD-Al2O3 interface modification reduces the density of acceptor-like states at the heterojunction resulting in reduced interface recombination and wider depletion width. Indications of increased VOC resulting from the modification of the heterojunction interface as a result of ALD-Al2O3 treatment are presented. These results, while not conclusive for application to state-of-the-art high efficiency CZTSSe devices, suggest the need for further studies as it is probable that interface recombination contributes to reduced VOC even in such devices.