Electroluminescence analysis of high efficiency Cu(In,Ga)Se2 solar cells

Abstract

We compare the electroluminescence (EL) of polycrystalline ZnO∕CdS∕Cu(In,Ga)Se2 heterojunction solar cells with similar band gaps but different open circuit voltages, indicating a difference in the electronic quality of the absorber. Temperature dependent EL measurements reveal that all cells feature transitions from donor-acceptor pair recombination at lower temperature to band to band recombination at higher temperatures. However, the less efficient cells show a longer transition range with donor-acceptor pair recombination still apparent at room temperature. We find further that the part of the room temperature spectrum that is due to band to band transitions in the respective cells is relatively broader than expected from a direct semiconductor with a homogeneous band gap. We analyze this spectral broadening by a model that accounts for band gap fluctuations of the absorber material. The experimental results show that the dominant part of this spectral broadening results from the intentional band gap grading and not from stochastic band gap fluctuations. We show further that the spectral EL emission is linked to the photovoltaic external quantum efficiency by electro-optical reciprocity. In a similar way, the external EL quantum efficiency is related to the open circuit voltage of the device. We verify experimentally that the difference between radiative and measured open circuit voltage determines the EL external quantum efficiency of the solar cell. The best Cu(In,Ga)Se2 solar cell reaches an external light emitting diode quantum efficiency of around 0.1%.

Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

“…V oc,rad (c-Si) = 864 mV, E g (c-Si) = 1.124 eV). 63,64 The non-radiative losses in organic solar cells are huge compared to the radiative losses (hence the low luminescence efficiency of the CT state) 42 but they are small compared to crystalline silicon solar cells, if we compare the difference between the peak of the EL and the V oc . This is probably due to the blend structure that separates electrons from holes and suppresses nongeminate recombination.…”

Influence of energetic disorder on electroluminescence emission in polymer:fullerene solar cells

“…Using the EQE and the EL, we can calculate the radiative open circuit voltage V oc,rad using the method presented in Ref. 55. For P3HT:PCBM the value is V oc,rad = 0.982 V, as compared to a measured open circuit voltage V oc = 0.600 V. If we assume that the characteristic tail slope for P3HT:PCBM is 37 meV, the Urbach energy in Fig.…”

“…23,55,56 The quantity used to measure the absolute amount of EL emission is the external light emitting diode (LED) quantum efficiency. Unlike the solar cell quantum efficiency (EQE and EQE EL ), the LED quantum efficiency Q LED is a scalar quantity that is defined as the ratio of radiative recombination current density J rad and the total recombination current J tot , i.e.,…”

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

“…[15][16][17][18][19][20][21] However, to investigate the junction quality of the semiconductors, electroluminescence (EL) proves to be useful. [22][23][24][25] Nevertheless, EL-spectroscopy the principle of which is the exact inverse of the photovoltaic principle has never been used for CZTSSe material systems so far, although there have been some reports for CIGS materials. 22,23 In the present study we have compared the charge carrier recombination process across the junction regions of CZTSSe and highly efficient CIGS solar cells.…”

“…[22][23][24][25] Nevertheless, EL-spectroscopy the principle of which is the exact inverse of the photovoltaic principle has never been used for CZTSSe material systems so far, although there have been some reports for CIGS materials. 22,23 In the present study we have compared the charge carrier recombination process across the junction regions of CZTSSe and highly efficient CIGS solar cells. For this purpose, room temperature EL and time resolved photoluminescence (TRPL) under different external bias have been investigated.…”