Bayesian Inference: OPV light-intensity dependant JV fits with SIMsalabim (fake data)

This notebook is a demonstration of how to fit light-intensity dependent JV curves with drift-diffusion models using the SIMsalabim package.

# Import necessary libraries
import warnings, os, sys, shutil
# remove warnings from the output
os.environ["PYTHONWARNINGS"] = "ignore"
warnings.filterwarnings(action='ignore', category=FutureWarning)
warnings.filterwarnings(action='ignore', category=UserWarning)
import pandas as pd
import matplotlib.pyplot as plt
import numpy as np
from numpy.random import default_rng
import torch, copy, uuid
import pySIMsalabim as sim
from pySIMsalabim.experiments.JV_steady_state import *
import ax, logging
from ax.utils.notebook.plotting import init_notebook_plotting, render
init_notebook_plotting() # for Jupyter notebooks

try:
    from optimpv import *
    from optimpv.axBOtorch.axUtils import *
except Exception as e:
    sys.path.append('../') # add the path to the optimpv module
    from optimpv import *
    from optimpv.axBOtorch.axUtils import *

[INFO 08-13 17:27:30] ax.utils.notebook.plotting: Injecting Plotly library into cell. Do not overwrite or delete cell.
[INFO 08-13 17:27:30] ax.utils.notebook.plotting: Please see
    (https://ax.dev/tutorials/visualizations.html#Fix-for-plots-that-are-not-rendering)
    if visualizations are not rendering.

Define the parameters for the simulation

params = [] # list of parameters to be optimized

mun = FitParam(name = 'l2.mu_n', value = 7e-8, bounds = [1e-9,1e-6], log_scale = True, value_type = 'float', fscale = None, rescale = False, display_name=r'$\mu_n$', unit='m$^2$ V$^{-1}$s$^{-1}$', axis_type = 'log', force_log = True)
params.append(mun)

mup = FitParam(name = 'l2.mu_p', value = 5e-8, bounds = [1e-9,1e-6], log_scale = True, value_type = 'float', fscale = None, rescale = False, display_name=r'$\mu_p$', unit=r'm$^2$ V$^{-1}$s$^{-1}$', axis_type = 'log', force_log = True)
params.append(mup)

bulk_tr = FitParam(name = 'l2.N_t_bulk', value = 1e20, bounds = [1e19,1e22], log_scale = True, value_type = 'float', fscale = None, rescale = False,  display_name=r'$N_{T}$', unit=r'm$^{-3}$', axis_type = 'log', force_log = True)
params.append(bulk_tr)

preLangevin = FitParam(name = 'l2.preLangevin', value = 1e-2, bounds = [0.005,1], log_scale = True, value_type = 'float', fscale = None, rescale = False, display_name=r'$\gamma_{pre}$', unit=r'', axis_type = 'log', force_log = True)
params.append(preLangevin)

R_series = FitParam(name = 'R_series', value = 1e-4, bounds = [1e-5,1e-3], log_scale = True, value_type = 'float', fscale = None, rescale = False,  display_name=r'$R_{series}$', unit=r'$\Omega$ m$^2$', axis_type = 'log', force_log = True)
params.append(R_series)

# save the original parameters for later
params_orig = copy.deepcopy(params)

Generate some fake data

Here we generate some fake data to fit. The data is generated using the same model as the one used for the fitting, so it is a good test of the fitting procedure. For more information on how to run SIMsalabim from python see the pySIMsalabim package.

# Set the session path for the simulation and the input files
session_path = os.path.join(os.path.join(os.path.abspath('../'),'SIMsalabim','SimSS'))
input_path = os.path.join(os.path.join(os.path.join(os.path.abspath('../'),'Data','simsalabim_test_inputs','fakeOPV')))
simulation_setup_filename = 'simulation_setup_fakeOPV.txt'
simulation_setup = os.path.join(session_path, simulation_setup_filename)

# path to the layer files defined in the simulation_setup file
l1 = 'ZnO.txt'
l2 = 'ActiveLayer.txt'
l3 = 'BM_HTL.txt'
l1 = os.path.join(input_path, l1)
l2 = os.path.join(input_path, l2)
l3 = os.path.join(input_path, l3)

# copy this files to session_path
force_copy = True
if not os.path.exists(session_path):
    os.makedirs(session_path)
for file in [l1,l2,l3,simulation_setup_filename]:
    file = os.path.join(input_path, os.path.basename(file))
    if force_copy or not os.path.exists(os.path.join(session_path, os.path.basename(file))):
        shutil.copyfile(file, os.path.join(session_path, os.path.basename(file)))
    else:
        print('File already exists: ',file)

# Show the device structure
fig = sim.plot_band_diagram(simulation_setup, session_path)

# reset simss
# Set the JV parameters
Gfracs = [0.1,0.5,1] # Fractions of the generation rate to simulate (None if you want only one light intensity as define in the simulation_setup file)
UUID = str(uuid.uuid4()) # random UUID to avoid overwriting files

cmd_pars = [] # see pySIMsalabim documentation for the command line parameters
# Add the parameters to the command line arguments
for param in params:
    cmd_pars.append({'par':param.name, 'val':str(param.value)})

# Run the JV simulation
ret, mess = run_SS_JV(simulation_setup, session_path, JV_file_name = 'JV.dat', G_fracs = Gfracs, parallel = True, max_jobs = 3, UUID=UUID, cmd_pars=cmd_pars)

# save data for fitting
X,y = [],[]
X_orig,y_orig = [],[]
if Gfracs is None:
    data = pd.read_csv(os.path.join(session_path, 'JV_'+UUID+'.dat'), sep=r'\s+') # Load the data
    Vext = np.asarray(data['Vext'].values)
    Jext = np.asarray(data['Jext'].values)
    G = np.ones_like(Vext)
    rng = default_rng()#
    noise = rng.standard_normal(Jext.shape) * 0.01 * Jext
    Jext = Jext + noise
    X = Vext
    y = Jext

    plt.figure()
    plt.plot(X,y)
    plt.show()
else:
    for Gfrac in Gfracs:
        data = pd.read_csv(os.path.join(session_path, 'JV_Gfrac_'+str(Gfrac)+'_'+UUID+'.dat'), sep=r'\s+') # Load the data
        Vext = np.asarray(data['Vext'].values)
        Jext = np.asarray(data['Jext'].values)
        G = np.ones_like(Vext)*Gfrac
        rng = default_rng()#
        noise = rng.standard_normal(Jext.shape) * 0.005 * Jext

        if len(X) == 0:
            X = np.vstack((Vext,G)).T
            y = Jext + noise
            y_orig = Jext
        else:
            X = np.vstack((X,np.vstack((Vext,G)).T))
            y = np.hstack((y,Jext+ noise))
            y_orig = np.hstack((y_orig,Jext))

    # remove all the current where Jext is higher than a given value
    X = X[y<200]
    X_orig = copy.deepcopy(X)
    y_orig = y_orig[y<200]
    y = y[y<200]



    plt.figure()
    for Gfrac in Gfracs:
        plt.plot(X[X[:,1]==Gfrac,0],y[X[:,1]==Gfrac],label='Gfrac = '+str(Gfrac))
    plt.xlabel('Voltage [V]')
    plt.ylabel('Current density [A/m$^2$]')
    plt.legend()
    plt.show()

Run the optimization

# Define the Agent and the target metric/loss function
from optimpv.DDfits.JVAgent import JVAgent
metric = 'mse' # can be 'nrmse', 'mse', 'mae'
loss = 'linear' # can be 'linear', 'huber', 'soft_l1'

# create a different params list for the agent
params_agent = copy.deepcopy(params)
#select a random value between the bounds, we do this because the walkers will be randomly initialized from the param.value
for param in params_agent:
    if param.force_log:
        param.value =10**np.random.uniform(np.log10(param.bounds[0]),np.log10(param.bounds[1]))
    else:
        param.value = np.random.uniform(param.bounds[0],param.bounds[1])

jv = JVAgent(params, X, y, session_path, simulation_setup, parallel = True, max_jobs = 3, metric = metric, loss = loss)

# Calulate the target metric for the original parameters
best_fit_possible = loss_function(calc_metric(y,y_orig, metric_name = metric),loss)
print('Best fit: ',best_fit_possible)

Best fit:  0.28005831028145817

MCMC Bayesian Inference for Parameter Fitting

We’ll use the emcee package to perform Markov Chain Monte Carlo sampling to find the posterior distribution of our model parameters.

from optimpv.BayesInfEmcee.EmceeOptimizer import EmceeOptimizer

# Define the Bayesian Inference object
optimizer = EmceeOptimizer(params = params, agents = jv, nwalkers=20, nsteps=2000, burn_in=100, progress=True, name='emcee_opti')

optimizer.optimize()

[INFO 2025-08-13 17:27:31,715] optimpv.BayesInfEmcee.EmceeOptimizer: Running MCMC with 20 walkers for 2000 steps...

----------------------------------------------------

100%|██████████| 100/100 [01:08<00:00,  1.45it/s]
100%|██████████| 2000/2000 [20:08<00:00,  1.66it/s]
[INFO 2025-08-13 17:48:51,437] optimpv.BayesInfEmcee.EmceeOptimizer: MCMC run complete.
[INFO 2025-08-13 17:48:51,448] optimpv.BayesInfEmcee.EmceeOptimizer: MCMC Results (Median & 16th/84th Percentiles)
[INFO 2025-08-13 17:48:51,449] optimpv.BayesInfEmcee.EmceeOptimizer: $\mu_n$ (l2.mu_n): 6.015e-08 (+6.34e-08 / -2.2e-08)
[INFO 2025-08-13 17:48:51,449] optimpv.BayesInfEmcee.EmceeOptimizer: $\mu_p$ (l2.mu_p): 5.533e-08 (+6.86e-08 / -2.42e-08)
[INFO 2025-08-13 17:48:51,449] optimpv.BayesInfEmcee.EmceeOptimizer: $N_{T}$ (l2.N_t_bulk): 4.212e+19 (+6.47e+19 / -2.46e+19)
[INFO 2025-08-13 17:48:51,450] optimpv.BayesInfEmcee.EmceeOptimizer: $\gamma_{pre}$ (l2.preLangevin): 0.009357 (+0.00381 / -0.00308)
[INFO 2025-08-13 17:48:51,450] optimpv.BayesInfEmcee.EmceeOptimizer: $R_{series}$ (R_series): 9.837e-05 (+4.77e-05 / -4.79e-05)

----------------------------------------------------

{'l2.mu_n': {'median': np.float64(6.014606890715596e-08),
  '16th': np.float64(3.816208452598251e-08),
  '84th': np.float64(1.2352463823526105e-07),
  'lower_err': np.float64(2.1983984381173457e-08),
  'upper_err': np.float64(6.337856932810508e-08)},
 'l2.mu_p': {'median': np.float64(5.532677125269291e-08),
  '16th': np.float64(3.1122211576392954e-08),
  '84th': np.float64(1.239355762742183e-07),
  'lower_err': np.float64(2.4204559676299956e-08),
  'upper_err': np.float64(6.860880502152538e-08)},
 'l2.N_t_bulk': {'median': np.float64(4.212239283604533e+19),
  '16th': np.float64(1.7476208366474541e+19),
  '84th': np.float64(1.0682424659536201e+20),
  'lower_err': np.float64(2.464618446957079e+19),
  'upper_err': np.float64(6.470185375931668e+19)},
 'l2.preLangevin': {'median': np.float64(0.009357205939363799),
  '16th': np.float64(0.006275504818845882),
  '84th': np.float64(0.013170601111692404),
  'lower_err': np.float64(0.003081701120517917),
  'upper_err': np.float64(0.003813395172328605)},
 'R_series': {'median': np.float64(9.836998305321582e-05),
  '16th': np.float64(5.045978659638413e-05),
  '84th': np.float64(0.00014606127717969037),
  'lower_err': np.float64(4.791019645683169e-05),
  'upper_err': np.float64(4.769129412647455e-05)}}

# Run simulation with best-fit parameters
best_fit_params = copy.deepcopy(optimizer.params)

# Create agent with best-fit parameters
best_agent = JVAgent(best_fit_params, X, y, session_path, simulation_setup,
                     parallel=True, max_jobs=3, metric=metric, loss=loss)

best_fit_y = best_agent.run(parameters={})

# Plot the best-fit curves against data
plt.figure(figsize=(10, 6))
for Gfrac in Gfracs:
    mask = X[:,1] == Gfrac
    plt.plot(X[mask,0], y[mask], 'ko', alpha=0.6, label=f'Data (G={Gfrac})')
    plt.plot(X[mask,0], best_fit_y[mask], 'k-', label=f'MCMC fit (G={Gfrac})')

# add the 10 best fit curves
for i in range(10):
    # prep dum dic
    dum_dic = {}
    # add the parameters
    idx = 0
    for param in params:
        if param.type != 'fixed':
            dum_dic[param.name] = optimizer.flat_samples[i][idx]
            idx += 1

    y_ = best_agent.run(parameters=dum_dic)
    for Gfrac in Gfracs:
        mask = X[:,1] == Gfrac
        plt.plot(X[mask,0], y_[mask], 'b-', alpha=0.3, zorder = -1)

plt.xlabel('Voltage [V]')
plt.ylabel('Current density [A/m$^2$]')
plt.legend()
plt.title('Best-fit JV Curves from MCMC')
plt.grid(True, alpha=0.3)
plt.show()

# corner plot
True_params = {p.name: p.value for p in params_orig} # Dictionary to hold true values for parameters
fig = optimizer.plot_corner(True_params=True_params) # Dictionary to hold true values for parameters

fig.set_size_inches(15, 15)

[WARNING 2025-08-13 17:48:54,815] root: Too few points to create valid contours

# optimizer.plot_trace()
fig = optimizer.plot_traces()
fig.set_size_inches(15, 15)

# Clean up the output files (comment out if you want to keep the output files)
sim.clean_all_output(session_path)
sim.delete_folders('tmp',session_path)
# uncomment the following lines to delete specific files
sim.clean_up_output('ZnO',session_path)
sim.clean_up_output('ActiveLayer',session_path)
sim.clean_up_output('BM_HTL',session_path)
sim.clean_up_output('simulation_setup_fakeOPV',session_path)