Supplementary Information

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1 DOI: 1.138/NPHOTON Supplementary Information Enhanced power conversion efficiency in polymer solar cells using an inverted device structure Zhicai He, Chengmei Zhong, Shijian Su, Miao Xu, Hongbin Wu*, and Yong Cao NATURE PHOTONICS 1

2 DOI: 1.138/NPHOTON X-ray photoelectron spectroscopy (XPS) measurement 5 4 Counts / s [E k - qv] (ev) Figure S1 X-ray photoelectron spectroscopy (XPS) measurement. The XPS secondary cut-off of ITO substrates with PFN layer or without thin film on top. The diamonds, circles and squares are for pristine ITO substrate, ITO coated with 1 nm (PFN) and ITO coated with 2 nm (PFN), respectively. The measured work functions are 4.7 ev, 4.1 ev and 3.9 ev for the pristine ITO substrate, ITO coated with 1 nm (PFN) and ITO coated with 2 nm (PFN), respectively. Due to its semiconducting nature, the 2 nm PFN interlayer only provides moderate device performance, while a thin layer of 5 nm (not shown here) could not provide an ohmic contact for photoactive layer. 2 NATURE PHOTONICS

3 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION Small angle X-ray diffraction (XRD) analysis 15 Counts (cps) theta (degree) Figure S2 X-ray diffraction (XRD). Small angle X-ray diffraction pattern of the PFN layer (2 nm) spin-casted on the cleaned ITO substrate surface. The XRD feature indicates the existence of an ordered structure in the PFN layer. NATURE PHOTONICS 3

4 DOI: 1.138/NPHOTON Device performance certification report by the National Center of Supervision & Inspection on Solar Photovoltaic Products Quality of China (CPVT), and NEWPORT Corporation Figure S3a, Cover page of test report by CPVT on PTB7 device 4 NATURE PHOTONICS

5 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION Figure S3b, Test results of the PTB7 device with an effective area of mm 2, by CPVT. NATURE PHOTONICS 5

6 DOI: 1.138/NPHOTON Figure S3c I-V, power output and J-V characteristics of the inverted structure PTB7 device with an effective area of mm 2, by CPVT. 6 NATURE PHOTONICS

7 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION Figure S3d, Cover page of test report by NEWPORT Corporation on PTB7 device. NATURE PHOTONICS 7

8 DOI: 1.138/NPHOTON Figure S3e, Test results of the PTB7 device with an effective area of 16.7 mm 2, by NEWPORT Corporation. 8 NATURE PHOTONICS

9 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION Stability comparison between the conventional and the inverted polymer solar cells stored in air under ambient conditions. a % 8.72% 8.2% 7.84% 8.64% 8.66% 8.6% PCE (%) % 1.67% Days b 2 Current density (ma cm -2 ) Device ID: H151-C, Day Device ID: H151-C, Day 15 Device ID: H151-C, Day 2 Device ID: H151-C, Day 4 Device ID: H151-C, Day Voltage (V) NATURE PHOTONICS 9

10 DOI: 1.138/NPHOTON Figure S4 Stability comparison between the conventional and the inverted polymer solar cells. a, The efficiency as a function of time for conventional device and inverted device stored in air under ambient conditions. The square and the circle legends are for the conventional device and the inverted structure device, respectively. b, The J-V characteristics as a function of time for an inverted device stored in air under ambient conditions. 1 NATURE PHOTONICS

11 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION The reflectance spectra, and the photos of the conventional and the inverted structure (ZnO, PFN) PTB7:PC71BM PSCs Figure S5 The reflectance spectra of the conventional and the inverted structure PTB7:PC71BM PSCs from 3 to 8 nm. The square, the circle and the diamonds legends are for the conventional, ZnO-inverted and PFN-inverted structure device, respectively. Inset shows the photo of the conventional (left) and the inverted device based on ZnO (middle) and PFN(right). The stronger absorption in the inverted device can also be experimentally verified by the reflectance spectra measurement on the real devices (see Fig. S5). The inverted devices showed a decreased reflectance in wide range of spectra (4-8 nm) as compared to the conventional device. Since the real devices are opaque with highly reflective top electrode, a reduced reflectance implies enhanced optical absorption and better utilization of incident photons. Consistent with the reflectance spectra, the NATURE PHOTONICS 11

12 DOI: 1.138/NPHOTON photographs of the real devices look different to a naked eye (see the inset of Fig. S5, the conventional device exhibiting a sky-blue colour, while the inverted device showing a dark-blue colour), confirming an enhanced optical absorption in the inverted polymer solar cells. The reflectance spectra of the ZnO device is also included for comparison, which shows a stronger reflectivity as compared with the PFN device. 12 NATURE PHOTONICS

13 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION Device performance dependence on the PFN thickness Current density (ma cm -2 ) a 5nm 1nm 2nm Voltage (V) 2 b Maximum J sc (ma cm -2 ) Inverted Thickness of PFN (nm) Figure S6 Device performance dependence on the PFN thickness. a. The experimental J V characteristics of inverted device with configuration of ITO/PFN (x=5, 1, 2 nm)/ptb7:pc 71 BM (8 nm)/moo 3 (1 nm)/al under AM 1.5G illumination. The circles, squares and diamonds are for x=5, 1, 2 nm, respectively. NATURE PHOTONICS 13

14 DOI: 1.138/NPHOTON b. The calculated maximum J sc under AM 1.5G illumination (44-8 nm) as a function of the interlayer thickness. Table S1 the dependence of the device performance on the thickness of the PFN layer (deduced from Figure S6). The Thickness of PFN (nm) PCE (%) J sc (ma cm -2 ) FF (%) V OC (V) NATURE PHOTONICS

15 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION The photocurrent density dependence on the incident light intensity of the conventional and the inverted structure PTB7:PC71BM PSCs a 5 Current density (ma cm -2 ) O.D=1.5 O.D=1. O.D=.8 O.D=.5 O.D=.3 O.D=.2 O.D=.1 O.D= Voltage (V) b Current density (ma cm -2 ) O.D= O.D=.1 O.D=.2 O.D=.3 O.D=.5 O.D=.8 O.D=1. O.D= Voltage (V) NATURE PHOTONICS 15

16 DOI: 1.138/NPHOTON c Photocurrent (ma cm -2 ) y = * x^(.85166) R 2 = Light Intensity (W m -2 ) d Photocurrent (ma cm -2 ) y = * x^(.85793) R 2 = Light Intensity (W m -2 ) Figure S7 the photocurrent density dependence on the incident light intensity. a-b, Photocurrent density versus voltage (J ph V) characteristics for PCDTBT:PC 71 BM solar cells with conventional (a) and inverted (b) device structure under AM 1.5 G illumination with a set of neutral density filters. c-d, the double-logarithmic plot of photocurrent density as a function of the incident light intensity for 16 NATURE PHOTONICS

17 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION PCDTBT:PC 71 BM solar cells with conventional (c) and inverted (d) device structure. The photocurrent density in the conventional device and inverted device give comparable dependence on incident light intensity. The solid lines in (c) and (d) represent the best fittings following J ph I. As shown in Fig. S7, photocurrent density in the conventional device and inverted device give comparable dependence on incident light intensity (with a slope of.852 and.858, R 2 =.997 and.998, respectively), further verifies similar charge carrier recombination losses in the both devices. Note that for the photocurrent measurement, the J V sweeps were repeated twice for good reproducibility. NATURE PHOTONICS 17

18 DOI: 1.138/NPHOTON Net photocurrent density versus applied voltage (Jph V) characteristics for the conventional device and the inverted devices Photocurrent density (ma cm -2 ) 1 J J sat sc P max Applied Voltage (V) Figure S8 The effect of the device structure on net photocurrent density versus applied voltage (J ph V) characteristics. Net photocurrent density versus applied voltage (J ph V) characteristics of the conventional devices (open black circle) and inverted structure device (solid red circle) under constant incident light intensity (AM 1.5G, 1 W m -2 ). Figure S8 depicted the net photocurrent density (J ph ) dependence on the applied voltage (V). We have examined the heating up effect on the photocurrent and found no big difference within 1 min continuous illumination (see Figure S1, Table S3). At large reverse voltage, since all of the photogenerated excitons are dissociated into free charge carriers and bimolecular recombination loss is minimized [1,2], Jph 18 NATURE PHOTONICS

19 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION saturates (at V= 1.3 V) for the both devices as expected. However, the saturation photocurrent density (Jsat) for the conventional device and the inverted device are 16.7 ma cm 2 and 18.6 ma cm 2, respectively. Since J sat is only governed by the amount of absorbed incident photon flux (Nphoton) [3, 4] thus the maximal obtainable exciton generation rates (Gmax, given by J sat eg max L, where L is the thickness of the active layer) in the two devices under AM 1.5G illumination can be determined. In addition, the efficiency of other processes for free photogenerated carriers in the inverted device is not compromised by the increase in photons harvest, which enables independent control of J sc, thus can be exploited to avoid the trade-off between FF and J sc in this study. Direct proof comes from the ratio between the experimental net photocurrent density and the observed saturation photocurrent density (J ph /J sat ) in the device, which is essentially the product of exciton dissociation efficiency and charge collection efficiency [3, 5]. Indeed, J ph /J sat in both devices is nearly the same over a wide range of applied voltage (Figure S7). For example, at the maximal power output condition (corresponding to V=.55.6 V for both devices), photocurrent density J ph is 92% and 93% of the saturation current density J sat for the conventional and the inverted device, respectively. While at the short current density condition, J ph /J sat ratio is 83% for both devices. Apparently, these results suggest that the exciton dissociation efficiency and the charge collection efficiency in the inverted device are retained as efficient as that in the conventional device [5]. NATURE PHOTONICS 19

20 DOI: 1.138/NPHOTON Device performance comparison between the best inverted polymer solar devices fabricated side-by-side using PFN and ZnO as electron selective electrode, respectively. 2 Current density (ma cm -2 ) ZnO PFN Voltage (V) Figure S9 The J-V characteristics of the best inverted polymer solar devices fabricated side-by-side using PFN and ZnO as electron selective electrode, respectively. The square and the circle legends are for the PFN, ZnO based inverted structure device, respectively. Table S2 the best device performance of the inverted polymer solar devices fabricated side-by-side using PFN and ZnO as electron selective electrode, respectively (deduced from Figure S9). ETL PCE (%) J sc (ma cm -2 ) FF (%) V OC (V) ZnO PFN NATURE PHOTONICS

21 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION J-V characteristics of the device obtained at t= and after illumination for 5 and 1 min Current density (ma cm -2 ) 1 5 mins 5 mins 1 mins Voltage (V) Figure S1 J-V characteristics of the device obtained at t= and after illumination for 5 and 1 min, respectively. Table S3 The device performance obtained at t= and after illumination for 5 and 1 min (deduced from Figure S1). Illumination Time (min.) PCE (%) Jsc (ma cm -2 ) FF (%) V OC (V) NATURE PHOTONICS 21

22 DOI: 1.138/NPHOTON Typical atomic force microscopy (AFM) topography images of the MoO 3 deposited atop the active layer Figure S11 Typical atomic force microscopy (AFM) topography images of the MoO 3 deposited atop the active layer. The films are smooth in general, with a root-mean-square (RMS) roughness of.48 nm. 22 NATURE PHOTONICS

23 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION The measured refractive index (n) and extinction coefficient (k) spectra (44 nm - 84 nm) 3 a Refractive Index nm 51 nm 2 nm Wavelength (nm) 2.5 b Refractive index Extinction coefficient Wavelength (nm) NATURE PHOTONICS 23

24 DOI: 1.138/NPHOTON Refractive index c Extinction coefficient Wavelength (nm) Figure S12 The measured refractive index (n) and extinction coefficient (k) spectra (44 nm - 84 nm) of MoO 3 (a), PFN (b) and the photoactive layer (PTB7:PC 71 BM=1:1.5 by weight) (c). Note that the extinction coefficients of MoO 3 is too small and was not included in (a). 24 NATURE PHOTONICS

25 DOI: 1.138/NPHOTON SUPPLEMENTARY INFORMATION References 1 Blom, P. W. M., Mihailetchi, V. D., Koster, L. J. A., & Markov, D. E. Device physics of polymer:fullerene bulk heterojunction solar cells, Adv. Mater. 19, (27). 2 Shuttle, C. G., Hamilton, R., O Regan, B. C., Nelson, J. & Durrant, J. R. Charge-density-based analysis of the current-voltage response of polythiophene/fullerene photovoltaic devices. Proc. Natl. Acad. Sci. USA 17, (21). 3 Mihailetchi, V. D., Koster, L. J. A., Hummelen. J. C. & Blom, P. W. M. Photocurrent generation in polymer-fullerene bulk heterojunctions. Phys. Rev. Lett. 93, (24). 4 Maturová, K., van Bavel, S. S., Wienk, M. M., Janssen, R. A. J. & Kemerink, M. Description of the morphology dependent charge transport and performance of polymer:fullerene bulk heterojunction solar cells, Adv. Funct. Mater. 21, (211). 5 He, Z. C., Zhong, C. M., Huang, X., Wong, W.-Y., Wu, H. B., Chen, L. W., Su, S. J. & Y. Cao. Simultaneous enhancement of open-circuit voltage, short-circuit current density and fill factor in polymer solar cells, Adv. Mater., 23, (211). NATURE PHOTONICS 25