The Optical Diode Ideality Factor Enables Fast Screening of Semiconductors for Solar Cells

In the search for new materials for solar cells, a fast feedback is needed. Radiative efficiency measurements based on photoluminescence PL are the tool of choice to screen the voltage a material is capable of. Additionally the dependence of the radiative efficiency on excitation density contains information on the diode ideality factor, which determines in turn the fill factor of the solar cell. Both parameters are immediate ingredients of the efficiency of a solar cell and can be determined from PL measurements, which allow fast feedback. The method to determine the optical diode ideality factor from PL measurements and compare to electrical measurements in finished solar cells are discusse

Quasi Fermi level splitting of Cu-rich and Cu-poor Cu(In,Ga)Se2 absorber layers

The quasi Fermi level splitting is measured for Cu(In,Ga)Se2 absorber layers with different copper to (indium + gallium) ratios and for different gallium contents in the range of 20%-40%. For absorbers with a [Cu]/[In + Ga] ratio below one, the measured quasi Fermi level splitting is 120 meV higher compared to absorbers grown under copper excess independent of the gallium content, contrary to the ternary CuInSe2 where the splitting is slightly higher for absorber layers grown under copper excess. Possible explanations are the gallium gradient determined by the secondary ion mass spectrometry measurement which is less pronounced towards the surface for stoichiometric absorber layers or a fundamentally different recombination mechanism in the presence of gallium. Comparing the quasi Fermi level splitting of an absorber to the open circuit voltage of the corresponding solar cell, the difference for copper poor cells is much lower (60 meV) than that for copper rich cells (140 meV). The higher loss in V OC in the case of the Cu-rich material is attributed to tunneling enhanced recombination due to higher band bending within the space charge region

The stability domain of the selenide kesterite photovoltaic materials and NMR investigation of the Cu/Zn disorder in Cu 2 ZnSnSe 4 (CZTSe)

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7.6% CZGSe Solar Cells Thanks to Optimized CdS Chemical Bath Deposition

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Numerical modelling of the performance-limiting factors in CZGSe solar cells

Numerical models are proposed that are able to describe the current-voltage (I-V) behaviour of two Cu2ZnGeSe4(CZGSe) solar cells measured under different illuminations at 300 K. In the model, the doping density of the CZGSe layer and the mobility of carriers are determined by capacitance-voltage (C-V) profiling and AC field Hall effect measurement, respectively. Some of the other parameters in the solar cells, including the series and the shunt resistance, the metal work function of the back contact, defect properties and absorption coefficient in the absorber layer, are determined using a differential evolution algorithm by fitting of the model with the experimentally measured I-V curves. Sensitivity analysis of our proposed model with SCAPS-1D demonstrates that the low metal work function, which results in a hole barrier at the back contact, the low shunt resistance and the high series resistance of the cell can explain the low fill factor and the low efficiency in these cells

Crystal Chemistry, Optical–Electronic Properties, and Electronic Structure of Cd 1– x In 2+2 x /3 S 4 Compounds (0 ≤ x ≤ 1), Potential Buffer in CIGS-Based Thin-Film Solar Cells

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What do we know about binary/ternary phases in kesterite thin-film solar cells?

Kesterite semiconductors are promising materials and potential candidates for future thin-film photovoltaic application. However, the efficiency of the solar cells based on these compounds is still below the record efficiency obtained with chalcopyrite based materials such as Cu(In,Ga)Se2. There are several factors behind the efficiency limitations in kesterite thin-film solar cells but the most detrimental factor is the presence of unfavorable binary and ternary phases in the absorber. The most commonly reported secondary phases in kesterite compounds are Zn(S)Se, Cu2(S)Se, Sn(S)Se2 and Cu2Sn(S)Se3, some were present even in devices with high efficiency and their effect on the device performance depends on their position in the absorber. In this work, secondary phases in kesterite absorbers Cu2ZnSnSe4, Cu2ZnGe(S)Se4 and Cu2ZnGeSe4 [1-2] prepared using different fabrication processes are investigated and analyzed using several characterization techniques, among others Fourier-transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). We found that the formation of the different secondary phases observed in these studied compounds strongly depends on the film growth conditions and device preparation. The impact of the different phases on the device performance, especially on the open-circuit voltage and efficiency, is discussed together with some suggested solutions to remove or reduce their formation at different parts of the absorber. In addition, the cell performance is analyzed using numerical modelling and the results obtained for pure germanium substitution compound Cu2ZnGeSe4 with a record efficiency of 8.5% [3], show that device efficiency up to 14% could be achieved if no binary/ternary phases were present in the absorber. Acknowledgements: The authors would like to acknowledge the SWInG project financed by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 640868 and the Research Foundation Flanders-Hercules Foundation (FWO Vlaanderen, project No AUGE/13/16:FT-IMAGER

Back and front contacts in kesterite solar cells : state-of-the-art and open questions

We review the present state-of-the-art within back and front contacts in kesterite thin film solar cells, as well as the current challenges. At the back contact, molybdenum (Mo) is generally used, and thick Mo(S, Se)2 films of up to several hundred nanometers are seen in record devices, in particular for selenium-rich kesterite. The electrical properties of Mo(S, Se)2 can vary strongly depending on orientation and indiffusion of elements from the device stack, and there are indications that the back contact properties are less ideal in the sulfide as compared to the selenide case. However, the electronic interface structure of this contact is generally not well-studied and thus poorly understood, and more measurements are needed for a conclusive statement. Transparent back contacts is a relatively new topic attracting attention as crucial component in bifacial and multijunction solar cells. Front illuminated efficiencies of up to 6% have so far been achieved by adding interlayers that are not always fully transparent. For the front contact, a favorable energy level alignment at the kesterite/CdS interface can be confirmed for kesterite absorbers with an intermediate [S]/([S]+[Se]) composition. This agrees with the fact that kesterite absorbers of this composition reach highest efficiencies when CdS buffer layers are employed, while alternative buffer materials with larger band gap, such as Cd1−x Zn x S or Zn1−x Sn x O y , result in higher efficiencies than devices with CdS buffers when sulfur-rich kesterite absorbers are used. Etching of the kesterite absorber surface, and annealing in air or inert atmosphere before or after buffer layer deposition, has shown strong impact on device performance. Heterojunction annealing to promote interdiffusion was used for the highest performing sulfide kesterite device and air-annealing was reported important for selenium-rich record solar cells

Unveiling the role of copper content in the crystal structure and phase stability of epitaxial Cu(In,Ga)S2 films on GaP/Si(001)

International audienceThis study examines the growth condition to obtain a single-phase Cu(In,Ga)S2 (CIGS) chalcopyrite film epitaxially grown by coevaporation on a GaP/Si(001) pseudo-substrate. In particular, we report the structural differences between KCN-etched Cu-rich and Cu-poor CIGS films coevaporated on GaP/Si(001) by 1-stage process. The Cu-poor CIGS film consists of at least three phases; the main crystal is found to be chalcopyrite-ordered, coexisting with In-rich CuIn5S8, and CuAu-ordered CuInS2, all sharing epitaxial relationships with each other and the GaP/Si(001) pseudo-substrate. On the other hand, the Cu-rich CIGS film is single-phase chalcopyrite and displays sharper X-ray diffraction peaks and a lower density of microtwin defects. The elimination of the secondary CuAu-ordered phase with Cu excess is demonstrated. In both films, the chalcopyrite crystal exclusively grows with its c-axis aligned with the out-of-plane direction of Si[001]. This study confirms prior findings on the thermodynamics of Cu-In-Ga-S and the stability of secondary phases