Dominant Recombination Path Research Articles

Post‐deposition annealing and interfacial atomic layer deposition buffer layers of Sb2Se3/CdS stacks for reduced interface recombination and increased open‐circuit voltages

AbstractCurrently, Sb2Se3 thin films receive considerable research interest as a solar cell absorber material. When completed into a device stack, the major bottleneck for further device improvement is the open‐circuit voltage, which is the focus of the work presented here. Polycrystalline thin‐film Sb2Se3 absorbers and solar cells are prepared in substrate configuration and the dominant recombination path is studied using photoluminescence spectroscopy and temperature‐dependent current–voltage characteristics. It is found that a post‐deposition annealing after the CdS buffer layer deposition can effectively remove interface recombination since the activation energy of the dominant recombination path becomes equal to the bandgap of the Sb2Se3 absorber. The increased activation energy is accompanied by an increased photoluminescence yield, that is, reduced non‐radiative recombination. Finished Sb2Se3 solar cell devices reach open‐circuit voltages as high as 485 mV. Contrarily, the short‐circuit current density of these devices is limiting the efficiency after the post‐deposition annealing. It is shown that atomic layer‐deposited intermediate buffer layers such as TiO2 or Sb2S3 can pave the way for overcoming this limitation.

Dominant recombination path in low-bandgap kesterite CZTSe(S) solar cells from red light induced metastability

Hetero-junction kesterite Cu2ZnSn(S,Se)4 solar cells with low bandgap obtained from three different methods of fabrication were exposed to red light illumination, and the changes observed in their electronic properties due to this exposure were studied via open circuit voltage transients, admittance spectroscopy, capacitance voltage profiling techniques, and SCAPS simulation fits to experimental data. The results from the aforementioned techniques, in combination with temperature-dependent current voltage analysis, can be used to reveal the dominant Shockley–Read–Hall recombination path at open circuit voltage. We also derived analytical expressions for the activation energy of the saturation current density and the diode quality factor for the specific case of a solar cell device that has no type inversion at the absorber/buffer interface and is limited by interface recombination in the open circuit condition. It is found that the dominant recombination pathway for the low bandgap Cu2ZnSn(S,Se)4 solar cells under consideration is located in the space charge region and not at the absorber/buffer interface.

Passivating Surface Defects and Reducing Interface Recombination in CuInS2 Solar Cells by a Facile Solution Treatment

Interface recombination at the absorber surface impedes the efficiency of a solar cell with an otherwise excellent absorber. The internal voltage or quasi‐Fermi‐level splitting (qFLs) measures the quality of the absorber. Interface recombination reduces the open‐circuit voltage (VOC) with respect to the qFLs. A facile solution‐based sulfur postdeposition treatment (S‐PDT) is explored to passivate the interface of CuInS2 grown under Cu‐rich conditions, which show excellent qFLs values, but much lower VOCs. The absorbers are treated in S‐containing solutions at 80 °C. Absolute calibrated photoluminescence and current–voltage measurements demonstrate a reduction of the deficit between qFLs and VOC by almost one‐third compared with the untreated device. Temperature dependence of the open‐circuit voltage shows increased activation energy for the dominant recombination path, indicating less interface recombination. In addition, capacitance transients reveal the presence of slow metastable defects in the untreated solar cell. The slow response is considerably reduced by the S‐PDT, suggesting passivation of these slow metastable defects. The results demonstrate the effectiveness of solution‐based S‐treatment in passivating defects, presenting a promising strategy to explore and reduce defect states near the interface of chalcogenide semiconductors.

Exciton-Exciton Annihilation in Two-Dimensional Halide Perovskites at Room Temperature.

Recently, Ruddlesden-Popper 2D perovskite (RPP) solar cells and light-emitting diodes (LEDs) have shown promising efficiencies and improved stability in comparison to 3D halide perovskites. Here, the exciton recombination dynamics is investigated at room temperature in pure-phase RPP crystals (C6H5C2H4NH3)2(CH3NH3)n-1PbnI3n+1 (n = 1, 2, 3, and 4) by time-resolved photoluminescence (TRPL) in a large range of power excitations. As the number of perovskite layers increases, we detect the presence of an increasing fraction of out-of-equilibrium free carriers just after photoexcitation, on a picosecond time scale, while the dynamics is characterized by the recombination of excitons with long lifetime spanning several tens of nanoseconds. At low excitation power, the TRPL decays are nonexponential because of defect-assisted recombination. At high fluence, defects are filled and many-body interactions become important. Similar to other 2D systems, exciton-exciton annihilation (EEA) is then the dominant recombination path in a high-density regime below the Mott transition.

Investigation of recombination mechanisms of CdTe solar cells with different buffer layers

It is critical to understand the dominant recombination mechanisms of thin film solar cells for further improving their performance. The dominant recombination paths of CdTe solar cells with MgxZn1-xO (MZO), MZO/CdS and MZO/CdS/CdSe buffer layers were investigated by temperature-dependent current-voltage (JVT) characterization both in light and dark, with traditional CdS/CdTe devices as a reference. The devices with MZO or MZO/CdS buffer layers are confirmed to be dominated by interface recombination, which leads to poor performance of devices. Compared to devices with traditional CdS buffer layers which are dominated by bulk recombination, the devices with MZO/CdS/CdSe buffer layer exhibit the same dominant recombination path but superior performance. It demonstrates that MZO/CdS/CdSe can be a promising composite buffer layer.

Improved open-circuit voltage in Cu(In,Ga)Se2 solar cells with high work function transparent electrodes

Hydrogenated indium oxide (IOH) is implemented as transparent front contact in Cu(In,Ga)Se2 (CIGS) solar cells, leading to an open circuit voltage VOC enhanced by ∼20 mV as compared to reference devices with ZnO:Al (AZO) electrodes. This effect is reproducible in a wide range of contact sheet resistances corresponding to various IOH thicknesses. We present the detailed electrical characterization of glass/Mo/CIGS/CdS/intrinsic ZnO (i-ZnO)/transparent conductive oxide (TCO) with different IOH/AZO ratios in the front TCO contact in order to identify possible reasons for the enhanced VOC. Temperature and illumination intensity-dependent current-voltage measurements indicate that the dominant recombination path does not change when AZO is replaced by IOH, and it is mainly limited to recombination in the space charge region and at the junction interface of the solar cell. The main finding is that the introduction of even a 5 nm-thin IOH layer at the i-ZnO/TCO interface already results in a step-like increase in VOC. Two possible explanations are proposed and verified by one-dimensional simulations using the SCAPS software. First, a higher work function of IOH as compared to AZO is simulated to yield an VOC increase by 21 mV. Second, a lower defect density in the i-ZnO layer as a result of the reduced sputter damage during milder sputter-deposition of IOH can also add to a maximum enhanced VOC of 25 mV. Our results demonstrate that the proper choice of the front TCO contact can reduce the parasitic recombination and boost the efficiency of CIGS cells with improved corrosion stability.

Recombination via point defects and their complexes in solar silicon

AbstractElectronic grade Czochralski and float zone silicon in the as grown state have a very low concentration of recombination generation centers (typically <1010 cm−3). Consequently, in integrated circuit technologies using such material, electrically active inadvertent impurities and structural defects are rarely detectable. The quest for cheap photovoltaic cells has led to the use of less pure silicon, multi‐crystalline material, and low cost processing for solar applications. Cells made in this way have significant extrinsic recombination mechanisms. In this paper we review recombination involving defects and impurities in single crystal and in multi‐crystalline solar silicon. Our main techniques for this work are recombination lifetime mapping measurements using microwave detected photoconductivity decay and variants of deep level transient spectroscopy (DLTS). In particular, we use Laplace DLTS to distinguish between isolated point defects, small precipitate complexes and decorated extended defects. We compare the behavior of some common metallic contaminants in solar silicon in relation to their effect on carrier lifetime and cell efficiency. Finally, we consider the role of hydrogen passivation in relation to transition metal contaminants, grain boundaries and dislocations. We conclude that recombination via point defects can be significant but in most multi‐crystalline material the dominant recombination path is via decorated dislocation clusters within grains with little contribution to the overall recombination from grain boundaries.

Importance of Recombination at the TCO/Electrolyte Interface for High Efficiency Quantum Dot Sensitized Solar Cells

Here, we show that, in CdSe quantum dot sensitized solar cells (QDSCs), recombination of electrons from the transparent conducting oxide (TCO) front electrode with oxidized species of the polysulfide redox electrolyte cannot be neglected like in dye-sensitized solar cells (DSCs). We demonstrate that light to electric power conversion efficiencies up to 4% can be achieved when recombination at the front electrode is suppressed by a compact TiO2 layer deposited in between the TCO substrate and the QD sensitized porous TiO2 film. Numerical simulations based on a simple equivalent circuit suggest that, over a wide potential range, electron transfer into the electrolyte at the TCO substrate is the dominant recombination path, which is usually not considered, suggesting that the current understanding of QDSCs has to be revised.

Use of different Zn precursors for the deposition of Zn(S,O) buffer layers by chemical bath for chalcopyrite based Cd‐free thin‐film solar cells

AbstractProgress in fabricating Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells with Zn(S,O) buffer layers prepared by chemical bath deposition (CBD) is discussed. The effect of different Zn salt precursors on solar cell device performance is investigated using production scale CIGSSe absorbers provided by AVANCIS GmbH & Co. KG. The CBD process has been developed at the Hahn‐Meitner‐Institut (HMI) using zinc nitrate, zinc sulphate or zinc chloride as zinc precursor. An average efficiency of 14.2 ± 0.8% is obtained by using one‐layer CBD Zn(S,O) The dominant recombination path for well performing solar cells is discussed based on the results obtained from temperature dependent J (V) analysis. The structure and morphology of buffer layers deposited using zinc nitrate and zinc sulphate has been studied by means of transmission electron micrographs of glass/Mo/CIGSSe/Zn(S,O) structures. Results show a conformal coverage of the absorber by a Zn(S,O) layer of 15–25 nm consisting of nanocrystals with radii of ∼5 nm. XAES analysis of the buffer layer reveals a similar surface composition for buffer layers deposited with zinc nitrate and zinc sulphate. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence from impurity codoped and compensated Si nanocrystals

Photoluminescence (PL) properties of B and P codoped and compensated Si nanocrystals were studied. The compensation of carriers in nanocrystals was confirmed by the annihilation of confined-carrier optical absorption in the infrared region. In the PL spectra obtained under the resonant excitation condition, the codoped samples did not exhibit structures related to momentum-conserving phonons, which were clearly observed for pure Si nanocrystals. The result strongly suggests that in impurity codoped Si nanocrystals, nonphonon quasidirect optical transition is the dominant recombination path for electron-hole pairs, and thus impurity codoping is a possible approach to further improving PL efficiency of Si nanocrystals.

Determination of dominant recombination paths in Cu(In,Ga)Se 2 thin-film solar cells with ALD–ZnO buffer layers

CuIn 1− x Ga x Se 2 (CIGS) and CuInSe 2 (CIS) thin-film solar cells, with ZnO buffer layers deposited by Atomic Layer Deposition (ALD), are examined with respect to dominant recombination path. They are compared with reference cells with CdS buffer layers. The principal method of examination is temperature-dependent J– V characterization ( J( V) T), and the analysis of the J( V) T data has been modified in order to more reliably discern the dominant recombination path. Compared to the CIS cells with the traditional CdS buffer layer, the CIS cells with ALD–ZnO buffer layer exhibit the same dominant recombination path, i.e., recombination in the bulk of the absorber. For the CIGS cells (with [Ga]/([Ga]+[In])=0.3), however, the analysis of the cells with ALD–ZnO buffer points to dominant interface recombination, while the CdS buffer cells are dominated by bulk recombination. For CIGS, the difference between the recombination in ALD–ZnO and CdS cells is consistent with the negative conduction band offset found by photoelectron spectroscopy in these ALD–ZnO cells in a previous study. This offset leads to increased interface recombination. For CIS/ALD–ZnO, it was previously found that there is no negative conduction band offset since the conduction band minimum of the absorber is lower. Consistently there is no difference in dominant recombination path between ALD–ZnO buffer cells and traditional CdS buffer cells.

Interdependence of absorber composition and recombination mechanism in Cu(In,Ga)(Se,S)2 heterojunction solar cells

Temperature-dependent current-voltage measurements are used to determine the dominant recombination path in thin-film heterojunction solar cells based on a variety of Cu(In,Ga)(Se,S)2 alloys. The activation energy of recombination follows the band gap energy of the respective Cu(In,Ga)(Se,S)2 alloy as long as the films are grown with a Cu-poor final composition. Thus, electronic loss in these devices is dominated by bulk recombination. In contrast, all devices based on absorber alloys with a Cu-rich composition prior to heterojunction formation are dominated by recombination at the heterointerface, with activation energies smaller than the band gap energy of the absorber material. These activation energies are independent from the S/Se ratio but increase with increasing Ga/In ratio.

Auger Recombination via Defects in Tellurium

AbstractAuger process including a bound electron and two free holes proved to be the dominant recombination path in tellurium at low temperatures (T < 50 K). The experimental value of the Auger constant is C = 1.6 × 10−28 cm6 s−1. The theoretical model considering the tellurium band structure explains the experimental data qualitatively and gives an order of magnitude value for the lifetimes of excess carriers.

Drift mobility in n– and p–conducting a-Si: H

Abstract The drift mobility of a series of undoped and doped a-Si: H films has been determined from their steady-state photoconductivity and response time. The drift mobilities at 300 K were 0·1 cm2 V−1 s−1 and 5 × 10−4 cm2 V−1 s−1 for electrons and holes, respectively. The activation energies were 0·13 and 0·27 eV. There is no influence of doping up to a doping level of 1000 ppm of either PH3 or B2H6. The temperature dependence of the response time reflects changes in the dominant recombination path. At low temperatures tunnelling processes prevail, but with increasing temperature direct capture of free carriers by dangling bonds becomes predominant.

Measurement of Free-Carrier Lifetimes in GaP by Photoinduced Modulation of Infrared Absorption

A new technique is presented for measuring the lifetimes of both minority and majority photoexcited free carriers in GaP. The technique is useful for both n- and p- type material in any doping range and involves measurement of the modulation of the free-carrier infrared absorption induced by photoexcited free carriers. In GaP(Zn, O) we find that the minority-carrier (electron) lifetime ranges from 1 to 20 nsec, and the majority-carrier (hole) lifetime ranges from 0.1 to 2 μ sec. A saturation effect is observed which is consistent with the view that the dominant recombination path in GaP(Zn, O) is through a deep acceptor via an Auger mechanism.