Decreasing Charge Losses in Perovskite Solar Cells Through the mp ...
Decreasing Charge Losses in Perovskite Solar Cells Through the mp-TiO2/MAPI Interface Engineering. a
a
Jose Manuel Marin-Beloqui, Luis Lanzetta and Emilio Palomares*
a,b
aInstitute of Chemical Research of Catalonia (ICIQ). Avda. Països Catalans, 16. Tarragona. E-
43007. Spain. bICREA. Passeig Lluís Companys 23. Barcelona. E-08010. Spain.
*To whom the correspondence should be addressed: Email: [email protected]
Supporting information.
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Figure 1S. PICE measurement for both MAPI perovskite solar cells extending the collection time over 1.5 seconds.
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Figure 2S. PICE decays for Al2O3 treated devices and standard mTiO2/MAPI perovskite solar cells.
Page 3 de 12
Figure 3S. Measured charge using PICE at different solar cell voltage from MAPI perovskite solar cells and Al2O3–coated perovskite solar cells.
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Figure 4S. Voc stability measurement previous PICE and PIT-PV measurements.
Page 5 de 12
Figure 5S. PICE and PIT-PV decays for a typical DSSC using iodine/iodide liquid electrolyte and 8μm thick mpTiO2.
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Photo Induced Charge Extraction (PICE) In PICE the device is held at open-circuit conditions while a bias (Voc) is obtained with illumination using a set of white light LEDs. This voltage can be adjusted varying the intensity of the applied light, reaching from more than 1 sun (100mW/cm2) to dark conditions. The LEDs are illuminating the device for a sufficient time to reach steady state conditions without thermal degradation problems and , thus, achieving a constant Voc. Thereafter, at the same time as the LEDs are switched off (the LEDs response is close to 300 ns, which denotes the resolution of the technique) the device is short-circuited, and charges pass through a known resistance resistor. The voltage decay is measured with a digital oscilloscope. This voltage decay is transformed by the Ohm’s law into current and then is integrated to obtain the total charges extracted (Equation 1) from the active layer of the device at each voltage point obtained by the use of different light bias intensity. In the end, varying the light intensity of the LEDs, a plot of the variation of the charge of the device with the applied voltage can be obtained. Q ⋅ 1 ⋅ Equation 1
Figure 4S. Scheme of the PICE system.
Photo-Induced Transient PhotoVoltage (PIT-PV) The Photo-Induced Transient PhotoVoltage (PIT-PV) is a technique used to obtain the lifetime of the charge carriers (τn) generated in the device at certain charge density. The diagram of the TPV setup is shown in Figure 4S as described in the bibliography1, 2. The device is connected to a high resistance to maintain open circuit conditions while a voltage is applied by illumination with a ring of LEDs. Once the steady state is reached, a small perturbation is made with a low intensity laser pulse, generating a little amount of extra charge carriers, creating a little voltage increase (∆V0). As the device is under open circuit conditions the excess charges can only recombine to go back to the steady state (original Voc) when the pulse finishes, and this recombination is recorded as a voltage decay with a digital oscilloscope. The wavelength used by the laser to create the small voltage perturbation in the device has
Page 7 de 12
to be near the maximum of the absorption peak of the absorber material to achieve a uniform generation along its thickness and simultaneously obtain an adequate signal. The perturbation must be of a few millivolts (less than 10 mV), very small compared with the Voc of hundreds of mV of the solar cell under illumination. The reason for this is to approach the pseudo first order regime of the voltage decay and obtain a mono-exponential fitting (Equation 2). The lifetime value of this monoexponential decay is the recombination time of the charges generated in the device. A graded neutral density filter is used to control the intensity of the pulse obtained from the laser to maintain the perturbation between 5 and 10 mV.
V ∆ Equation 2
Figure 5S. Scheme of the PIT-PV system.
Photo-Induced Transient PhotoCurrent (PIT-PC) The Photo-Induced Transient PhotoCurrent (PIT-PC) is a technique where it is possible to obtain the charges produced in the device by a certain laser pulse. In this technique, the cell is connected to the oscilloscope going through by a known low resistance resistor while the cell is short-circuited. The use of a small resistance is needed to be capable of measuring a signal which is produced with changes in voltages as low as 10 mV. Upon the laser pulse on the device, it creates a perturbation in the current, which can be integrated to obtain the charge created by the laser pulse, ∆Q0. The measured charge is correct if we assume that there are negligible charge carrier losses at short circuit. To ensure that this is the case we must check: (a) that the TPC decay under dark and under light irradiation is almost alike. If the decays are similar it is feasible that the recombination is very small and, thus, negligible and, (b) moreover, the TPC decay does not depend on the background illumination intensity. (c) the device short circuit current is linear with increasing light intensity. This is important to rule out that non-geminate recombination at short-circuit is not an issue.
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Figure 6S. Scheme of a Transient PhotoCurrent setup.
Photo-Induced Differential Charging (PIDC) The Photo-Induced Differential Charging (PIDC) is a method that combines TPC and TPV in order to obtain the capacitance of photovoltaic devices where the PICE extraction is slower or in the same range than PIT-PV, what makes impossible the extraction of the total charges before they recombine. As mentioned above, to combine the use of PIT-PC and PIT-PV three rules must be followed, to be sure that there will be no losses at short-circuit and all the charges will be extracted. TPC decay must be the same under various light intensities. This means that there are no electric fields in the device. TPC decay must be faster than TPV at 1 sun. A linear relationship between Jsc and light intensity must exist (α=1). If only one of these assumptions is not true, the differential charging cannot be used to obtain the charge in the device. The PIDC is based on Equation 3 where the usual capacitance formula was translated in terms of values that can be calculated with TPV and TPC. Equation 3 ∆Q0 is the charge calculated in TPC, that remains constant when the V is changed. ∆V0 is the amplitude of the TPV signal at certain Voc obtained with the same laser intensity that TPC was performed. With the different amplitudes for different voltages, the capacitance distribution with the voltage is obtained. Cap
∆Q ∆V Equation 3
To obtain the charge distribution with voltage, the Equation 4 has to be followed. The charge at V=V’ will be the integral of the capacitance from 0 to V=V’. "#
Q ! Equation 4
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Cell
Cell 2
Cell 5
Al Coat
Yes
No
Voc (mV)
Q (nC/cm2) PICE
25000
PIDC
60
PICE
17000
PIDC
40
950
870
τ (µs) Slow Average Fast Slow Average Fast Slow Average Fast Slow Average Fast
44 12 1.5 44 12 1.5 44 12 0.6 44 12 0.6
OF
Jrec (mA/cm2)
4.9 4.6 2.7 2.3 2.2 1.7 4 3.3 2.2 2.8 2.1 1.9
93 450 9000 0.4 2.0 31 97 396 13000 0.3 1.4 32
Table S1. Charge in device measured with PICE and PIDC, different lifetime recombinations measured by PIT-PV, and Jrec calculated for MAPI perovskite solar cells and Al2O3-coated devices.
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Figure 7S. The HRTEM images for (top) non coated nanocrystalline TiO2 nanoparticles and (bottom) the Al2O3 conformally coated nanocrystalline TiO2 nanoparticles. The thin Al2O3 coating (about 1nm thick ) has been blue colourised ( dashed) to help the picture visualisation.
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REFERENCES. 1. A. Maurano, C. G. Shuttle, R. Hamilton, A. M. Ballantyne, J. Nelson, W. Zhang, M. Heeney and J. R. Durrant. Transient Optoelectronic Analysis of Charge Carrier Losses in a Selenophene/Fullerene Blend Solar Cell. J. Phys. Chem. C, 2011, 115, 5947-5957. 2.
J. W. Ryan, J. M. Marin-Beloqui, J. Albero and E. Palomares. Nongeminate Recombination Dynamics-Device Voltage Relationship in Hybrid PbS Quantum Dot/C60 Solar Cells. J. Phys. Chem. C, 2013, 117, 17470-17476.
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a
Jose Manuel Marin-Beloqui, Luis Lanzetta and Emilio Palomares*
a,b
aInstitute of Chemical Research of Catalonia (ICIQ). Avda. Països Catalans, 16. Tarragona. E-
43007. Spain. bICREA. Passeig Lluís Companys 23. Barcelona. E-08010. Spain.
*To whom the correspondence should be addressed: Email: [email protected]
Supporting information.
Page 1 de 12
Figure 1S. PICE measurement for both MAPI perovskite solar cells extending the collection time over 1.5 seconds.
Page 2 de 12
Figure 2S. PICE decays for Al2O3 treated devices and standard mTiO2/MAPI perovskite solar cells.
Page 3 de 12
Figure 3S. Measured charge using PICE at different solar cell voltage from MAPI perovskite solar cells and Al2O3–coated perovskite solar cells.
Page 4 de 12
Figure 4S. Voc stability measurement previous PICE and PIT-PV measurements.
Page 5 de 12
Figure 5S. PICE and PIT-PV decays for a typical DSSC using iodine/iodide liquid electrolyte and 8μm thick mpTiO2.
Page 6 de 12
Photo Induced Charge Extraction (PICE) In PICE the device is held at open-circuit conditions while a bias (Voc) is obtained with illumination using a set of white light LEDs. This voltage can be adjusted varying the intensity of the applied light, reaching from more than 1 sun (100mW/cm2) to dark conditions. The LEDs are illuminating the device for a sufficient time to reach steady state conditions without thermal degradation problems and , thus, achieving a constant Voc. Thereafter, at the same time as the LEDs are switched off (the LEDs response is close to 300 ns, which denotes the resolution of the technique) the device is short-circuited, and charges pass through a known resistance resistor. The voltage decay is measured with a digital oscilloscope. This voltage decay is transformed by the Ohm’s law into current and then is integrated to obtain the total charges extracted (Equation 1) from the active layer of the device at each voltage point obtained by the use of different light bias intensity. In the end, varying the light intensity of the LEDs, a plot of the variation of the charge of the device with the applied voltage can be obtained. Q ⋅ 1 ⋅ Equation 1
Figure 4S. Scheme of the PICE system.
Photo-Induced Transient PhotoVoltage (PIT-PV) The Photo-Induced Transient PhotoVoltage (PIT-PV) is a technique used to obtain the lifetime of the charge carriers (τn) generated in the device at certain charge density. The diagram of the TPV setup is shown in Figure 4S as described in the bibliography1, 2. The device is connected to a high resistance to maintain open circuit conditions while a voltage is applied by illumination with a ring of LEDs. Once the steady state is reached, a small perturbation is made with a low intensity laser pulse, generating a little amount of extra charge carriers, creating a little voltage increase (∆V0). As the device is under open circuit conditions the excess charges can only recombine to go back to the steady state (original Voc) when the pulse finishes, and this recombination is recorded as a voltage decay with a digital oscilloscope. The wavelength used by the laser to create the small voltage perturbation in the device has
Page 7 de 12
to be near the maximum of the absorption peak of the absorber material to achieve a uniform generation along its thickness and simultaneously obtain an adequate signal. The perturbation must be of a few millivolts (less than 10 mV), very small compared with the Voc of hundreds of mV of the solar cell under illumination. The reason for this is to approach the pseudo first order regime of the voltage decay and obtain a mono-exponential fitting (Equation 2). The lifetime value of this monoexponential decay is the recombination time of the charges generated in the device. A graded neutral density filter is used to control the intensity of the pulse obtained from the laser to maintain the perturbation between 5 and 10 mV.
V ∆ Equation 2
Figure 5S. Scheme of the PIT-PV system.
Photo-Induced Transient PhotoCurrent (PIT-PC) The Photo-Induced Transient PhotoCurrent (PIT-PC) is a technique where it is possible to obtain the charges produced in the device by a certain laser pulse. In this technique, the cell is connected to the oscilloscope going through by a known low resistance resistor while the cell is short-circuited. The use of a small resistance is needed to be capable of measuring a signal which is produced with changes in voltages as low as 10 mV. Upon the laser pulse on the device, it creates a perturbation in the current, which can be integrated to obtain the charge created by the laser pulse, ∆Q0. The measured charge is correct if we assume that there are negligible charge carrier losses at short circuit. To ensure that this is the case we must check: (a) that the TPC decay under dark and under light irradiation is almost alike. If the decays are similar it is feasible that the recombination is very small and, thus, negligible and, (b) moreover, the TPC decay does not depend on the background illumination intensity. (c) the device short circuit current is linear with increasing light intensity. This is important to rule out that non-geminate recombination at short-circuit is not an issue.
Page 8 de 12
Figure 6S. Scheme of a Transient PhotoCurrent setup.
Photo-Induced Differential Charging (PIDC) The Photo-Induced Differential Charging (PIDC) is a method that combines TPC and TPV in order to obtain the capacitance of photovoltaic devices where the PICE extraction is slower or in the same range than PIT-PV, what makes impossible the extraction of the total charges before they recombine. As mentioned above, to combine the use of PIT-PC and PIT-PV three rules must be followed, to be sure that there will be no losses at short-circuit and all the charges will be extracted. TPC decay must be the same under various light intensities. This means that there are no electric fields in the device. TPC decay must be faster than TPV at 1 sun. A linear relationship between Jsc and light intensity must exist (α=1). If only one of these assumptions is not true, the differential charging cannot be used to obtain the charge in the device. The PIDC is based on Equation 3 where the usual capacitance formula was translated in terms of values that can be calculated with TPV and TPC. Equation 3 ∆Q0 is the charge calculated in TPC, that remains constant when the V is changed. ∆V0 is the amplitude of the TPV signal at certain Voc obtained with the same laser intensity that TPC was performed. With the different amplitudes for different voltages, the capacitance distribution with the voltage is obtained. Cap
∆Q ∆V Equation 3
To obtain the charge distribution with voltage, the Equation 4 has to be followed. The charge at V=V’ will be the integral of the capacitance from 0 to V=V’. "#
Q ! Equation 4
Page 9 de 12
Cell
Cell 2
Cell 5
Al Coat
Yes
No
Voc (mV)
Q (nC/cm2) PICE
25000
PIDC
60
PICE
17000
PIDC
40
950
870
τ (µs) Slow Average Fast Slow Average Fast Slow Average Fast Slow Average Fast
44 12 1.5 44 12 1.5 44 12 0.6 44 12 0.6
OF
Jrec (mA/cm2)
4.9 4.6 2.7 2.3 2.2 1.7 4 3.3 2.2 2.8 2.1 1.9
93 450 9000 0.4 2.0 31 97 396 13000 0.3 1.4 32
Table S1. Charge in device measured with PICE and PIDC, different lifetime recombinations measured by PIT-PV, and Jrec calculated for MAPI perovskite solar cells and Al2O3-coated devices.
Page 10 de 12
Figure 7S. The HRTEM images for (top) non coated nanocrystalline TiO2 nanoparticles and (bottom) the Al2O3 conformally coated nanocrystalline TiO2 nanoparticles. The thin Al2O3 coating (about 1nm thick ) has been blue colourised ( dashed) to help the picture visualisation.
Page 11 de 12
REFERENCES. 1. A. Maurano, C. G. Shuttle, R. Hamilton, A. M. Ballantyne, J. Nelson, W. Zhang, M. Heeney and J. R. Durrant. Transient Optoelectronic Analysis of Charge Carrier Losses in a Selenophene/Fullerene Blend Solar Cell. J. Phys. Chem. C, 2011, 115, 5947-5957. 2.
J. W. Ryan, J. M. Marin-Beloqui, J. Albero and E. Palomares. Nongeminate Recombination Dynamics-Device Voltage Relationship in Hybrid PbS Quantum Dot/C60 Solar Cells. J. Phys. Chem. C, 2013, 117, 17470-17476.
Page 12 de 12
