Takuya Kato

Lifetime improvement for high efficiency Cu 2 ZnSnS 4 submodules

Photoluminescence (PL) lifetime of 36 ns on Cu₂ZnSnS₄ absorber was achieved by optimizing absorber formation. The high quality absorber enabled us to achieve 9.2% efficiency Cu₂ZnSnS₄ submodule. Comparison between the PL lifetime and atomic composition showed lower Cu/Sn ratio resulted in longer lifetime. The lower Cu/Sn ratio made bandgap higher and voltage deficit lower. The PL intensity mapping accelerated the quality clarification of the various absorbers and contributed to the achievement of high efficiency Cu₂ZnSnS₄ submodules.

Recent R&D progress in solar frontier's small-sized Cu(InGa)(SeS) 2 solar cells

Our resent achievement of the record breaking 20.9% efficiency (independently confirmed by Fraunhofer ISE) with small-sized CIS-based solar cell will be discussed in this paper by means of comparison with our previous result of 19.7%. The new record was mainly achieved by an improved Jsc arising from modified depth profile of the absorber layer and the doping concentration of the transparent conductive oxide (TCO) layer. Combination of these two modifications drastically enhances light absorption at a longer wavelength region, leading to Jsc improvement of about 3 mA/cm2 without losing Voc nor FF. This new 20.9% efficiency record resulted from a CIGS cell cut from a 30 cm by 30 cm substrate produced using the same method and materials we use in our factories, sputtering-selenization formation method with Cd-free buffer layer.

Efficiency improvement of Cu 2 ZnSn(S,Se) 4 submodule with graded bandgap and reduced backside ZnS segregation

A new record conversion efficiency of 11.0% was achieved on a Cu₂ZnSn(S,Se)₄ (CZTS) submodule by introducing a bandgap grading. The S/(S+Se) compositional ratio was graded from the bottom (higher S/(S+Se)) to the top (lower S/(S+Se)) of the CZTS absorber layer to modify the conduction band minimum. The submodule with the S-graded CZTS layer exhibited V_oc and J_sc improvements, and V_oc × J_sc was increased from 16.8 mW/cm² in our previous best submodule (η = 10.8%) to 17.6 mW/cm² (η = 11.0%). However, in most cases, the S grading was found to be accompanied with a ZnS segregation at the backside of the absorber. The backside ZnS layer increases the series resistance, and this is the reason that our new record submodule has higher V_oc × J_sc but lower fill factor. To decrease the series resistance, we have tried to reduce the ZnS segregation, with maintaining the S grading, by adjusting the precursor and selenization/sulfurization conditions. Finally, an S-graded CZTS absorber layer with less ZnS segregation has been successfully acquired, and the recovery of the series resistance has been confirmed. Further improvement of the efficiency will be achieved by optimizing and applying these techniques to current record submodule.

Determination of deep-level defects in Cu2ZnSn(S,Se)4 thin-films using photocapacitance method

Deep-level defects were investigated in Cu2ZnSn(S,Se)4 and Cu2ZnSnS4 thin-films using transient photocapacitance (TPC) spectroscopy. A deep-defect, OH1 centered around 1.0u2009eV above the valance-band (EV) of Cu2ZnSnS4 has been identified at room temperature (RT). However, OH1-defect could be identified in Cu2ZnSn(S,Se)4 at low temperature only. Absence of OH1-defect in Cu2ZnSn(S,Se)4 at RT explains its better performance comparing to Cu2ZnSnS4 solar-cell. A comparative study of the TPC spectra of the Cu(In,Ga)Se2 solar-cells was performed. Low intensity of defect-signal together with lower broadening of exponential band-tail in the TPC spectra were attributed to superior performance of Cu(In,Ga)Se2 solar-cells comparing to Cu2ZnSn(S,Se) counterpart.

Study of time-resolved photoluminescence in Cu2ZnSn(S,Se)4 thin films with different Cu/Sn ratio

To determine the minority carrier lifetime, room temperature time-resolved photoluminescence (TR-PL) measurements have been performed on a set of Cu2ZnSn(S,Se)4 (CZTSSe) samples with different Cu/Sn ratios of 1.65, 1.75, and 1.85. TR-PL measurements were carried out on the bare CZTSSe thin films, CdS covered CZTSSe films and on solar cell structure using a femtosecond laser. The sample containing high Cu/Sn ratio of 1.85 shows the lowest lifetime, while films with Cu/Sn ratios of 1.65 and 1.75 show almost equal lifetime. The difference in lifetime between the CdS covered and solar structure samples is not remarkable. This demonstrates domination of recombination than charge separation by electric field. The bare films show extremely small lifetime. To examine surface quality, TR-PL emission spectra of uncovered CZTSSe and Cu(In,Ga)Se2 (CIGS) films were measured with two different excitation wavelengths of 420 and 750 nm, which generate excess carriers at different depths in absorbers. This comparison confirms the dominant surface recombination by CZTSSe than CIGS.

A comparative study on charge carrier recombination across the junction region of Cu2ZnSn(S,Se)4 and Cu(In,Ga)Se2 thin film solar cells

A comparative study with focusing on carrier recombination properties in Cu2ZnSn(S,Se)4 (CZTSSe) and the CuInGaSe2 (CIGS) solar cells has been carried out. For this purpose, electroluminescence(EL) and also bias-dependent time resolved photoluminescence(TRPL) using femtosecond (fs) laser source were performed. For the similar forward current density, the EL-intensity of the CZTSSe sample was obtained significantly lower than that of the CIGS sample. Primarily, it can be attributed to the existence of excess amount of non-radiative recombination center in the CZTSSe, and/or CZTSSe/CdS interface comparing to that of CIGS sample. In case of CIGS sample, TRPL decay time was found to increase with the application of forward-bias. This can be attributed to the reduced charge separation rate resulting from the reduced electric-field at the junction. However, in CZTSSe sample, TRPL decay time has been found almost independent under the forward and reverse-bias conditions. This phenomenon indicates that the charge recombination rate strongly dominates over the charge separation rate across the junction of the CZTSSe sample. Finally, temperature dependent VOC suggests that interface related recombination in the CZTSSe solar cellstructure might be one of the major factors that affect EL-intensity and also, TRPL decay curves.

Study of Cu2ZnSn(S,Se)4 Thin Films for Solar Cell Application

Cu2ZnSn(S,Se)4 (CZTSSe) thin films with various Cu/Sn ratio in the films have been investigated to study the effect of compositional variation over the electrical, optical, and structural properties of the film. Surface morphology and grain size were found to be significantly influenced by the Cu/Sn ratio in the films and grain size was found better for the samples with moderate Cu/Sn ratio of 1.75. Irrespective of the growth condition and compositional variation, all the CZTSSe crystals show that grains are oriented along (112) direction as evident from the room temperature XRD data. Dark current-voltage (I-V) curve reveals that that sample with Cu/Sn = 1.75 exhibits lowest leakage current, while sample with Cu/Sn = 1.85 has the highest leakage current along with larger ideality factor indicating larger recombination centers in this film. Series resistance was also found to be higher in the sample with higher Cu-content. An anomaly in the optical band-gap has been explained with the presences of impurity phases and compositional inhomogeneities in the CZTSSe materials.

Defect study of Cu 2 ZnSn(S,Se) 4 thin film with different Cu/Sn ratio by admittance spectroscopy

Defect properties of Cu₂ZnSn(S_x,Se_1-x)₄ (CZTSSe) were investigated by admittance spectroscopy (AS). Two defect states (labeled E_A1 and E_A2) were observed in CZTSSe (x=0.15) with different Cu/Sn ratio. When the Cu/Sn ratio increased from 1.75 to 1.95, the activation energy of E_A1 and E_A2 decreased and the defect densities increased. The capture cross sections of E_A1 and E_A2 defects are in the order from 10^-16 cm² to 10^-18 cm², indicating that these two defects possibly do not impact on device performance.

Study of recombination process in Cu 2 ZnSnS 4 thin film using two-wavelength excited photoluminescence

Room-temperature two-wavelength excited photoluminescence (PL) measurements have been performed in the kesterite Cu2ZnSnS4(CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) thin film absorbers. A defect level at 0.8 eV from the valence band and its properties are investigated. Two light sources of 635nm and 1550nm diode lasers, respectively, were used for above bandgap and 0.8eV defect level excitation. The two-wavelength excited PL intensity was stronger than that only above-gap laser irradiation for the CZTS specimen. This phenomenon strongly suggests that the 0.8eV defect level acts as recombination center at room temperature. On the other hand, this defect may act as a trap in lower gap CZTSSe.